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>
(cherry picked from commit a744064c)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/tiff.c

@@ -79,6 +79,7 @@ static void free_geotags(TiffContext *const s)
             av_freep(&s->geotags[i].val);
     }
     av_freep(&s->geotags);
+    s->geotag_count = 0;
 }
 
 #define RET_GEOKEY(TYPE, array, element)\