eacmv: check the framerate before setting it.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org

libavcodec/eacmv.c

@@ -131,7 +131,7 @@ static void cmv_decode_inter(CmvContext *s, AVFrame *frame, const uint8_t *buf,
 
 static int cmv_process_header(CmvContext *s, const uint8_t *buf, const uint8_t *buf_end)
 {
-    int pal_start, pal_count, i, ret;
+    int pal_start, pal_count, i, ret, fps;
 
     if(buf_end - buf < 16) {
         av_log(s->avctx, AV_LOG_WARNING, "truncated header\n");
@@ -145,8 +145,9 @@ static int cmv_process_header(CmvContext *s, const uint8_t *buf, const uint8_t *
     if (ret < 0)
         return ret;
 
-    s->avctx->time_base.num = 1;
-    s->avctx->time_base.den = AV_RL16(&buf[10]);
+    fps = AV_RL16(&buf[10]);
+    if (fps > 0)
+        s->avctx->time_base = (AVRational){ 1, fps };
 
     pal_start = AV_RL16(&buf[12]);
     pal_count = AV_RL16(&buf[14]);