Add a check for c->head being NULL in pullup_free_context().

This fixes crashes when an invalid filter chain is built
Patch by Alexander Strange % astrange A ithinksw P com %
Original thread:
date: Sep 7, 2007 8:47 PM	 
subject: [MPlayer-dev-eng] [PATCH] crash in pullup with invalid filters


git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@24370 b3059339-0415-0410-9bf9-f77b7e298cf2

libmpcodecs/pullup.c

@@ -795,6 +795,7 @@ void pullup_free_context(struct pullup_context *c)
 	free(c->buffers);
 	f = c->head;
 	do {
+		if (!f) break;
 		free(f->diffs);
 		free(f->comb);
 		f = f->next;