Fix segfault when encoder initialization fails.

Patch by Craig Thomasson $(name) dot $(surname) ripcode com

Originally committed as revision 22811 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/utils.c

@@ -710,7 +710,7 @@ av_cold int avcodec_close(AVCodecContext *avctx)
         avctx->codec->close(avctx);
     avcodec_default_free_buffers(avctx);
     av_freep(&avctx->priv_data);
-    if(avctx->codec->encode)
+    if(avctx->codec && avctx->codec->encode)
         av_freep(&avctx->extradata);
     avctx->codec = NULL;
     entangled_thread_counter--;