fix segfault with http://sam.zoy.org/zzuf/lol-ffplay.ogm and

http://sam.zoy.org/zzuf/lol-ffplay.ogg

Originally committed as revision 7561 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/imgresample.c

@@ -672,6 +672,8 @@ struct SwsContext *sws_getContext(int srcW, int srcH, int srcFormat,
 
 void sws_freeContext(struct SwsContext *ctx)
 {
+    if (!ctx)
+        return;
     if ((ctx->resampling_ctx->iwidth != ctx->resampling_ctx->owidth) ||
         (ctx->resampling_ctx->iheight != ctx->resampling_ctx->oheight)) {
         img_resample_close(ctx->resampling_ctx);