avcodec/mpeg4videodec: set field durations to safe values when they are invalid

Fixes division by 0
Fixes: 3c980274cecaca6ca5d6e517c0c1983e-signal_sigabrt_7ffff7125425_410_cov_3621042467_2_audio_streams.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpeg4videodec.c

@@ -2297,8 +2297,10 @@ static int decode_vop_header(Mpeg4DecContext *ctx, GetBitContext *gb)
                             ROUNDED_DIV(s->last_non_b_time - s->pp_time, ctx->t_frame)) * 2;
         s->pb_field_time = (ROUNDED_DIV(s->time, ctx->t_frame) -
                             ROUNDED_DIV(s->last_non_b_time - s->pp_time, ctx->t_frame)) * 2;
-        if (!s->progressive_sequence) {
-            if (s->pp_field_time <= s->pb_field_time || s->pb_field_time <= 1)
+        if (s->pp_field_time <= s->pb_field_time || s->pb_field_time <= 1) {
+            s->pb_field_time = 2;
+            s->pp_field_time = 4;
+            if (!s->progressive_sequence)
                 return FRAME_SKIPPED;
         }
     }