h264dec: make sure to only end a field if it has been started

Calling ff_h264_field_end() when the per-field state is not properly
initialized leads to all kinds of undefined behaviour.

CC: libav-stable@libav.org
Bug-Id: 977 978 992

libavcodec/h264_picture.c

@@ -194,6 +194,7 @@ int ff_h264_field_end(H264Context *h, H264SliceContext *sl, int in_setup)
     emms_c();
 
     h->current_slice = 0;
+    h->field_started = 0;
 
     return err;
 }

libavcodec/h264_slice.c

@@ -1884,9 +1884,8 @@ int ff_h264_queue_decode_slice(H264Context *h, const H2645NAL *nal)
                 sl = h->slice_ctx;
             }
 
-            if (h->current_slice && h->cur_pic_ptr && FIELD_PICTURE(h)) {
+            if (h->field_started)
                 ff_h264_field_end(h, sl, 1);
-            }
 
             h->current_slice = 0;
             if (!h->first_field) {
@@ -1902,6 +1901,7 @@ int ff_h264_queue_decode_slice(H264Context *h, const H2645NAL *nal)
             ret = h264_field_start(h, sl, nal);
             if (ret < 0)
                 return ret;
+            h->field_started = 1;
         }
     }
 

libavcodec/h264dec.c

@@ -757,6 +757,7 @@ out:
 
     if (!(avctx->flags2 & AV_CODEC_FLAG2_CHUNKS) ||
         (h->mb_y >= h->mb_height && h->mb_height)) {
+        if (h->field_started)
             ff_h264_field_end(h, &h->slice_ctx[0], 0);
 
         *got_frame = 0;

libavcodec/h264dec.h

@@ -509,6 +509,11 @@ typedef struct H264Context {
      * slices) anymore */
     int setup_finished;
 
+    /* This is set to 1 if h264_field_start() has been called successfully,
+     * so all per-field state is properly initialized and we can decode
+     * the slice data */
+    int field_started;
+
     AVFrame *output_frame;
 
     int enable_er;