avcodec/movtextdec: Check style_start/end

Limits based on 3GPP TS 26.245 V14.0.0
Fixes: Timeout
Fixes: 6377/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MOVTEXT_fuzzer-5175929115508736

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Philip Langdale <philipl@overt.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/movtextdec.c

@@ -299,6 +299,14 @@ static int decode_styl(const uint8_t *tsmb, MovTextContext *m, AVPacket *avpkt)
         m->s_temp->style_start = AV_RB16(tsmb);
         tsmb += 2;
         m->s_temp->style_end = AV_RB16(tsmb);
+
+        if (   m->s_temp->style_end < m->s_temp->style_start
+            || (m->count_s && m->s_temp->style_start < m->s[m->count_s - 1]->style_end)) {
+            av_freep(&m->s_temp);
+            mov_text_cleanup(m);
+            return AVERROR(ENOMEM);
+        }
+
         tsmb += 2;
         m->s_temp->style_fontID = AV_RB16(tsmb);
         tsmb += 2;