Transaction limit 0 doesn't make any sense, enforce -L > 0

Also do not assert on empty incomplete list even though VTX count is
larger than L. This can happen when a complete multi-transaction
result is still unreported.

Original patch by Geoff Simmons (Uplex)

lib/libvarnishapi/vsl_arg.c

@@ -343,7 +343,7 @@ VSL_Arg(struct VSL_data *vsl, int opt, const char *arg)
 			p++;
 		if (*p != '\0')
 			return (vsl_diag(vsl, "-L: Syntax error"));
-		if (l < 0 || l > INT_MAX)
+		if (l <= 0 || l > INT_MAX)
 			return (vsl_diag(vsl, "-L: Range error"));
 		vsl->L_opt = (int)l;
 		return (1);

lib/libvarnishapi/vsl_dispatch.c

@@ -1344,7 +1344,8 @@ VSLQ_Dispatch(struct VSLQ *vslq, VSLQ_dispatch_f *func, void *priv)
 	}
 
 	/* Check store limit */
-	while (vslq->n_outstanding > vslq->vsl->L_opt) {
+	while (vslq->n_outstanding > vslq->vsl->L_opt &&
+	    !(VTAILQ_EMPTY(&vslq->incomplete))) {
 		vtx = VTAILQ_FIRST(&vslq->incomplete);
 		CHECK_OBJ_NOTNULL(vtx, VTX_MAGIC);
 		vtx_force(vslq, vtx, "store overflow");