h264: Do not treat the initial frame special in handling of frame gaps

The not handling of frame gaps has lead to the lack of a dummy reference
frame, which has lead to the failure of decode_slice_header() which has
lead to one SEI recovery message being skiped which had introduced a
slightly suboptimal recovery point for at least 1 h264 file compared to
JM.

Found-by: Carl & BugMaster
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -3616,7 +3616,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
     } else {
         /* Shorten frame num gaps so we don't have to allocate reference
          * frames just to throw them away */
-        if (h->frame_num != h->prev_frame_num && h->prev_frame_num >= 0) {
+        if (h->frame_num != h->prev_frame_num) {
             int unwrap_prev_frame_num = h->prev_frame_num;
             int max_frame_num         = 1 << h->sps.log2_max_frame_num;
 
@@ -3689,7 +3689,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
             }
         }
 
-        while (h->frame_num != h->prev_frame_num && h->prev_frame_num >= 0 && !h0->first_field &&
+        while (h->frame_num != h->prev_frame_num && !h0->first_field &&
                h->frame_num != (h->prev_frame_num + 1) % (1 << h->sps.log2_max_frame_num)) {
             Picture *prev = h->short_ref_count ? h->short_ref[0] : NULL;
             av_log(h->avctx, AV_LOG_DEBUG, "Frame num gap %d %d\n",