convert process hangs forever if the source file does not exist

Problem

_imagemagick_convert() calls drupal_realpath() on the passed-in $source and $destination.

drupal_realpath() can return FALSE in some conditions, however, and this condition is never checked. One such condition is if the source file is not present which can sometimes happen in dev-stage-prod scenarios. If this happens _imagemagick_convert() proceeds to call the convert binary which simply hangs forever causing the site, as well, to hang forever.

Proposed resolution

Early return if drupal_realpath() returns FALSE.