Don't return a free'd pointer, when a file cannot be opened.

(closes issue #13462)
 Reported by: wackysalut


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@142740 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/file.c

@@ -938,6 +938,7 @@ struct ast_filestream *ast_readfile(const char *filename, const char *type, cons
 			ast_log(LOG_WARNING, "Unable to open %s\n", fn);
 			if (fs)
 				ast_free(fs);
+			fs = NULL;
 			if (bfile)
 				fclose(bfile);
 			free(fn);