Patch for Red Hat bug #892228

Hi,

see https://bugzilla.redhat.com/show_bug.cgi?id=892228
for backtrace. This happens when I start darktable while I have my MP3 player connected via USB. gphoto2 first thinks that it's a camera but than decides that there's something wrong and tries to call the GPContextErrorFunc and darktable crashes.
The patch fixes the problem for me.