meta: Fix a crash when calling lsmash_count_itunes_metadata.
meta: Fix a crash when calling lsmash_count_itunes_metadata.

Forgot to check 'udta' exsistence.

silverfilain/L-SMASH,maki-rxrz/L-SMASH,canbal/l-smash,dwbuiten/l-smash,silverfilain/L-SMASH,l-smash/l-smash,maki-rxrz/L-SMASH,mstorsjo/l-smash,silverfilain/L-SMASH,l-smash/l-smash,l-smash/l-smash,canbal/l-smash,mstorsjo/l-smash,dwbuiten/l-smash