decode: avoid leaks on failure in ff_get_buffer()

If the get_buffer() call fails, the frame might have some side data
already set. Make sure it gets freed.

CC: libav-stable@libav.org

libavcodec/decode.c

@@ -1069,6 +1069,9 @@ end:
         frame->height = avctx->height;
     }
 
+    if (ret < 0)
+        av_frame_unref(frame);
+
     return ret;
 }