avformat/mvdec: Check size for validity in var_read_string()

Fixes out of array read
Fixes: asan_heap-oob_49b1e5_12_011.movie
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/mvdec: Check size in read_table() for validity

This check is redundant with the previous commit but it provides
better error messages and feedback while the previous commit
ensures that var_read_string() doesnt return uninitialized arrays
if it itself is feeded with an invalid size possibly through a
different future codepath.

Fixes: asan_heap-oob_49b1e5_12_011.movie
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>