Fix a memory leak in ng_get_string_token. A dynamically-allocated

buffer wasn't freed if the function failed.

MFC after:	3 days

sys/netgraph/ng_parse.c

@@ -1762,6 +1762,7 @@ ng_get_string_token(const char *s, int *startp, int *lenp, int *slenp)
 			strcpy(p, v);
 		}
 	}
+	FREE(cbuf, M_NETGRAPH_PARSE);
 	return (NULL);		/* no closing quote */
 }