rv34: check that subsequent slices have the same type as first one.

This prevents some crashes when corrupted bitstream reports e.g. P-type
slice in I-frame. Official RealVideo decoder demands all slices to be
of the same type too.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit 23a1f0c592)

libavcodec/rv34.c

@@ -1269,6 +1269,13 @@ static int rv34_decode_slice(RV34DecContext *r, int end, const uint8_t* buf, int
             r->next_pts = r->cur_pts;
         }
         s->mb_x = s->mb_y = 0;
+    } else {
+        int slice_type = r->si.type ? r->si.type : FF_I_TYPE;
+
+        if (slice_type != s->pict_type) {
+            av_log(s->avctx, AV_LOG_ERROR, "Slice type mismatch\n");
+            return AVERROR_INVALIDDATA;
+        }
     }
 
     r->si.end = end;