Fixed PHP-1090: Dereference null return value

Fixes several coverity issues around php_mongo_runcommand() retval changes
that was introduced by the fix for PHP-1036 (1831821f69d87f2ea0b7f0eb16ca8a750d4ecc85)
Merge pull request #692 from bjori/coverity-runcommand

Fixed several coverity issues around php_mongo_runcommand() retval changes

* bjori/coverity-runcommand:
  Fixed PHP-1090: Dereference null return value