avcodec/movtextenc: Fix memleak on (re)allocation error

avcodec/movtextenc: Fix memleak on (re)allocation error

Up until now, the mov_text encoder used the dynamic array API for its
list of style attributes; it used the (horrible) av_dynarray_add() which
works with an array of pointers; on error it frees its array but not
the buffers referenced by the pointers said array contains. It also
returns no error code, encouraging not to check for errors.

These properties imply that this function may only be used if the buffers
referenced by the list either need not be freed at all or if they are
freed by other means (i.e. if the list contains non-ownership pointers).

In this case, the style attributes are owned by the pointers of the
dynamic list. Ergo the old style attributes leak on a subsequent
reallocation failure. But given that the (re)allocation isn't checked
for success, the style attribute intended to be added to the list also
leaks because the only pointer to it gets overwritten in the belief that
it is now owned by the list.

This commit fixes this by switching to av_fast_realloc() and an array
containing the styles directly instead of pointers to individually
allocated style attributes. The current style attributes are now no longer
individually allocated, instead they are part of the context.

Furthermore, av_fast_realloc() allows to easily distinguish between
valid and allocated elements, thereby allowing to reuse the array
(which up until now has always been freed after processing an
AVSubtitleRect).

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