Fix a corner case in sws emulation

Originally committed as revision 5827 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/imgresample.c

@@ -756,6 +756,9 @@ int sws_scale(struct SwsContext *ctx, uint8_t* src[], int srcStride[],
             res = -1;
             goto the_end;
         }
+    } else if (resampled_picture != &dst_pict) {
+        img_copy(&dst_pict, resampled_picture, current_pix_fmt,
+                        ctx->resampling_ctx->owidth, ctx->resampling_ctx->oheight);
     }
 
 the_end: