Make use of opendir() more consistent

When transferring a site recently, I saw an issue where a while(readdir()) loop didn't exit because the results of opendir() weren't checked; if the directory doesn't exist then readdir() doesn't have a handle to read from.

Pretty sure this was in CRM_Utils_File::getFilesByExtension(), may be reproducible by removing / relocating the extensions directory (or some other expected directory).