Properly handle uninitialized connection

The initial problem, when I create a second connections:

    Warning: pg_close() expects parameter 1 to be resource, null given in Pomm/Connection/Connection.php line 147

This probably silenced the real problem, but I don’t understand what happens.
Without this patch, the tests passed but there is some warning. I have no idea how to test (except playing with the ``ob_*`` functions).

    PHPUnit 4.4-ga1e1dd6 by Sebastian Bergmann.

    Configuration read from /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/tests/phpunit.travis.xml

    ..............................SS................................. 65 / 78 ( 83%)
    .............

    Time: 12.46 seconds, Memory: 10.00Mb

    OK, but incomplete, skipped, or risky tests!
    Tests: 78, Assertions: 508, Skipped: 2.
    PHP Warning:  pg_close() expects parameter 1 to be resource, null given in /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/Pomm/Connection/Connection.php on line 151
    PHP Stack trace:
    PHP   1. {main}() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/vendor/phpunit/phpunit/phpunit:0
    PHP   2. PHPUnit_TextUI_Command::main() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/vendor/phpunit/phpunit/phpunit:56
    PHP   3. PHPUnit_TextUI_Command->run() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/vendor/phpunit/phpunit/src/TextUI/Command.php:138
    PHP   4. Pomm\Connection\Connection->__destruct() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/Pomm/Connection/Connection.php:0
    PHP   5. pg_close() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/Pomm/Connection/Connection.php:151

    Warning: pg_close() expects parameter 1 to be resource, null given in /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/Pomm/Connection/Connection.php on line 151

    Call Stack:
        0.0002     228272   1. {main}() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/vendor/phpunit/phpunit/phpunit:0
        0.0037     566160   2. PHPUnit_TextUI_Command::main() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/vendor/phpunit/phpunit/phpunit:56
        0.0038     566784   3. PHPUnit_TextUI_Command->run() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/vendor/phpunit/phpunit/src/TextUI/Command.php:138
    12.4641    9566176   4. Pomm\Connection\Connection->__destruct() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/Pomm/Connection/Connection.php:0
    12.4641    9566224   5. pg_close() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/Pomm/Connection/Connection.php:151

    PHP Warning:  pg_close() expects parameter 1 to be resource, null given in /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/Pomm/Connection/Connection.php on line 151
    PHP Stack trace:
    PHP   1. {main}() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/vendor/phpunit/phpunit/phpunit:0
    PHP   2. PHPUnit_TextUI_Command::main() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/vendor/phpunit/phpunit/phpunit:56
    PHP   3. PHPUnit_TextUI_Command->run() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/vendor/phpunit/phpunit/src/TextUI/Command.php:138
    PHP   4. Pomm\Connection\Connection->__destruct() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/Pomm/Connection/Connection.php:0
    PHP   5. pg_close() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/Pomm/Connection/Connection.php:151

    Warning: pg_close() expects parameter 1 to be resource, null given in /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/Pomm/Connection/Connection.php on line 151

    Call Stack:
        0.0002     228272   1. {main}() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/vendor/phpunit/phpunit/phpunit:0
        0.0037     566160   2. PHPUnit_TextUI_Command::main() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/vendor/phpunit/phpunit/phpunit:56
        0.0038     566784   3. PHPUnit_TextUI_Command->run() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/vendor/phpunit/phpunit/src/TextUI/Command.php:138
    12.4644    9566176   4. Pomm\Connection\Connection->__destruct() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/Pomm/Connection/Connection.php:0
    12.4644    9566224   5. pg_close() /home/sanpi/projects/pomm-profiler/vendor/pomm/pomm/Pomm/Connection/Connection.php:151