avcodec/movtextdec: Fix leaks of strings upon reallocation failure

avcodec/movtextdec: Fix leaks of strings upon reallocation failure

Up until now, the 3GPP Timed Text decoder used av_dynarray_add()
for a list of font entries, a structure which contains an allocated
string. The font entries are owned by the pointers in the dynamic array
and are therefore unsuitable for av_dynarray_add() which simply frees
the array, but not the font entries and of course not the strings. The
latter all leak if reallocating the dynamic array fails.

This commit fixes this. It stops reallocating the array altogether:
After all, the final number of elements (pending errors) is already
known in advance.

Furthermore, the font entries are now the entries of the new array,
i.e. the font entries are no longer allocated separately. This also
removes one level of indirection.

Reviewed-by: Philip Langdale <philipl@overt.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>