avcodec/dvdsubdec: fix accessing dangling pointers

dvdsub_decode() can call append_to_cached_buf() 2 times, the second time
with ctx->buf as argument. If the second append_to_cached_buf() reallocs
ctx->buf, the argument will be a pointer to the previous, freed block.
This can cause invalid reads at least with some fuzzed files - and
possibly with valid files.

Since packets can apparently not be larger than 64K (even if packets are
combined), just use a fixed size buffer. It will be allocated as part of
the DVDSubContext, and although some memory is "wasted", it's relatively
minimal by modern standards and should be acceptable.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>