avcodec/tiff: reset geotag_count in free_geotags()

Fixes null pointer dereference
Fixes: signal_sigsegv_19d922e_3688_cov_1577641655_aletrek_tiff.mov
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
