dvbsubdec: Fixed segfault when decoding subtitles

This fixes a segfault (originally found in Movian, but traced to libav)
when decoding subtitles because only an array of rects is allocated,
but not the actual structs it contains. The issue was probably
introduced in commit 2383323 where the loop to allocate the rects in
the array was thrown away.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
