avcodec/microdvddec: Check for string end in 'P' case

Fixes out of array read
Fixes: a9502b60f4cecc19475382aee255f73c/asan_heap-oob_1e87fba_2548_a8ad47f6dde36644fe9cdc444d4632d0.sub

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
