MethodDescriptor: Fatal error: Call to a member function getParent()

If the `$node` is not an instance of `Descriptor\MethodDescriptor`, then `__invoke()` should return false.

While trying to build documentation, I am getting this error:

```
Load cache                                                         ..    0.024s
Preparing template "clean"                                         ..    0.008s
Preparing 16 transformations                                       ..    0.000s
Build "elements" index                                             ..    0.001s
Replace textual FQCNs with object aliases                          ..    0.009s
Resolve @link and @see tags in descriptions                        ..    0.001s
Build "packages" index                                             ..    0.002s
Collect all markers embedded in tags                               ..    0.000s
Build "namespaces" index and add namespaces to "elements"          ..    0.001s
Transform analyzed project into artifacts                          .. PHP Fatal error:  Call to a member function getParent() on a non-object in /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Transformer/Router/UrlGenerator/Standard/MethodDescriptor.php on line 36
[2014-07-13 01:57:12] phpDocumentor.ALERT: Fatal Error (E_ERROR): Call to a member function getParent() on a non-object {"file":"/srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Transformer/Router/UrlGenerator/Standard/MethodDescriptor.php","line":36} []
<!DOCTYPE html>
<html lang="en">
<head>
```
SeeAssembler: PHP Notice:  Uninitialized string offset: 0 for $reference

Sometimes `$reference[0]` is not set.

Here is the stacktrace
```
phpdoc -c phpdoc.xml
Collecting files .. OK
Initializing parser .. OK
Parsing files
Parsing [REDACTED]
Parsing [REDACTED]
PHP Notice:  Uninitialized string offset: 0 in /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Descriptor/Builder/Reflector/Tags/SeeAssembler.php on line 51
PHP Stack trace:
PHP   1. {main}() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/bin/phpdoc:0
PHP   2. phpDocumentor\Application->run() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/bin/phpdoc:41
PHP   3. Symfony\Component\Console\Application->run() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Application.php:186
PHP   4. Symfony\Component\Console\Application->doRun() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/symfony/console/Symfony/Component/Console/Application.php:124
PHP   5. Symfony\Component\Console\Application->doRunCommand() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/symfony/console/Symfony/Component/Console/Application.php:193
PHP   6. Symfony\Component\Console\Command\Command->run() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/symfony/console/Symfony/Component/Console/Application.php:887
PHP   7. phpDocumentor\Command\Project\RunCommand->execute() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/symfony/console/Symfony/Component/Console/Command/Command.php:252
PHP   8. Symfony\Component\Console\Command\Command->run() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Command/Project/RunCommand.php:253
PHP   9. phpDocumentor\Parser\Command\Project\ParseCommand->execute() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/symfony/console/Symfony/Component/Console/Command/Command.php:252
PHP  10. phpDocumentor\Parser\Parser->parse() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Parser/Command/Project/ParseCommand.php:205
PHP  11. phpDocumentor\Parser\Parser->parseFileIntoDescriptor() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Parser/Parser.php:319
PHP  12. phpDocumentor\Parser\File->parse() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Parser/Parser.php:359
PHP  13. phpDocumentor\Descriptor\ProjectDescriptorBuilder->buildFileUsingSourceData() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Parser/File.php:67
PHP  14. phpDocumentor\Descriptor\ProjectDescriptorBuilder->buildDescriptor() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Descriptor/ProjectDescriptorBuilder.php:99
PHP  15. phpDocumentor\Descriptor\Builder\Reflector\FileAssembler->create() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Descriptor/ProjectDescriptorBuilder.php:130
PHP  16. phpDocumentor\Descriptor\Builder\Reflector\FileAssembler->addClasses() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Descriptor/Builder/Reflector/FileAssembler.php:65
PHP  17. phpDocumentor\Descriptor\ProjectDescriptorBuilder->buildDescriptor() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Descriptor/Builder/Reflector/FileAssembler.php:137
PHP  18. phpDocumentor\Descriptor\Builder\Reflector\ClassAssembler->create() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Descriptor/ProjectDescriptorBuilder.php:130
PHP  19. phpDocumentor\Descriptor\Builder\Reflector\ClassAssembler->addConstants() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Descriptor/Builder/Reflector/ClassAssembler.php:58
PHP  20. phpDocumentor\Descriptor\ProjectDescriptorBuilder->buildDescriptor() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Descriptor/Builder/Reflector/ClassAssembler.php:77
PHP  21. phpDocumentor\Descriptor\Builder\Reflector\ConstantAssembler->create() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Descriptor/ProjectDescriptorBuilder.php:130
PHP  22. phpDocumentor\Descriptor\Builder\Reflector\AssemblerAbstract->assembleDocBlock() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Descriptor/Builder/Reflector/ConstantAssembler.php:43
PHP  23. phpDocumentor\Descriptor\ProjectDescriptorBuilder->buildDescriptor() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Descriptor/Builder/Reflector/AssemblerAbstract.php:40
PHP  24. phpDocumentor\Descriptor\Builder\Reflector\Tags\SeeAssembler->create() /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Descriptor/ProjectDescriptorBuilder.php:130
```
Merge pull request #1342 from gammamatrix/patch-2

MethodDescriptor: Fatal error: Call to a member function getParent()