[GH-184] Fix bug in classExists when using multiple autoloaders (i.e. ZF2)

This issue is created automatically through a Github pull request on behalf of bramstroker:

Url: https://github.com/doctrine/common/pull/184

Message:

The autoloading routine in the classExists method makes the assumption the autoload method always returns a value (i.e. no void) when the autoloading of a class is succesfull. When you have multiple autoloaders registered to the spl stack this can lead to problems. In my case I'm using ZF2 ClassmapAutoloader and StandardAutoloader as fallback. The classmapAutoloader returns nothing so the autoload method in the StandardAutoloader is called as well, because the for loop is continued. This results in a second include, and a fatal error "Cannot redeclare class" is triggered.