PHP-1231: PHP crashed when using selectCollection inside a Generator

https://jira.mongodb.org/browse/PHP-1231
@bjori pointed out a case where `MongoClient::selectCollection('','');` results in two exceptions:

```
Fatal error: Uncaught exception 'MongoException' with message 'MongoDB::__construct(): invalid name ' in /home/bjori/Sources/mongo-php-driver/bug.php:4
Stack trace:
#0 /home/bjori/Sources/mongo-php-driver/bug.php(4): MongoClient->selectCollection('', '')
#1 /home/bjori/Sources/mongo-php-driver/bug.php(9): a()
#2 {main}

Next exception 'MongoException' with message 'The MongoDB object has not been correctly initialized by its constructor' in /home/bjori/Sources/mongo-php-driver/bug.php:4
Stack trace:
#0 /home/bjori/Sources/mongo-php-driver/bug.php(0): MongoClient->selectCollection()
#1 /home/bjori/Sources/mongo-php-driver/bug.php(9): a()
#2 {main}
  thrown in /home/bjori/Sources/mongo-php-driver/bug.php on line 4
```

From: http://pastebin.com/tzj05VeF

Currently, full name validation is only done in the DB construct function. We should check if construction failed and avoid returning an uninitialized MongoDB object.