Ignore index lines without either a '+' or '-'

When upgrading from when a previous release that doesn't have the latest
VSM index file changes, the utilities will assert when trying to parse the
index file from the previous version. Fix that by ignoring lines not
starting with either '#', '+' or '-'.

lib/libvarnishapi/vsm.c

@@ -579,7 +579,7 @@ vsm_vlu_func(void *priv, const char *line)
 {
 	struct vsm *vd;
 	struct vsm_set *vs;
-	int i = -1;
+	int i = 0;
 
 	CAST_OBJ_NOTNULL(vs, priv, VSM_SET_MAGIC);
 	vd = vs->vsm;