Small leak and ineffective code GIOGetMimeType()

Interesting result from valgrind revealed that
 - a line of code *was* being executed,
 - but that that line's work was being *ignored*

Fix changes the line to be an assignment, and then adds the required intermediate g_free().

This typo didn't alter the final result, as the third (!) call to routine g_content_type_guess() gave the correct final result.  (I checked)

Still, @khaledhosny might want to check the original code to see if this typo exists there also.