avcodec/vorbisdec: Fix off by 1 error in ptns_to_read

Fixes read of uninitialized memory
Fixes: asan_heap-uaf_18dac2b_9_asan_heap-uaf_22eb375_208_beta3_test_small.ogg

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>