avcodec/utils: Check that the data is complete in avpriv_bprint_to_extradata()

Fixes out of array read
Fixes: asan_heap-oob_4d2250_814_cov_2745172097_JACOsub_capability_tester.jss
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>