Need to free() GIOGetMimeType() results before returning. See issue #1433

Converted GIOGetMimeType() to use basic malloc/free instead of possibly
more complex (unknown?) g_free() before returning results. GImageRead()
converted to free() mime results before continuing on with next step.
Free "mime" memory in GFileChooserDefFilter(). see issue #1433
Merge remote-tracking branch 'upstream/master'
Merge pull request #1473 from JoesCat/master

Free memory in GFileChooserDefFilter, GImageRead, and GIOGetMimeType.