vcc: Fix null pointer dereference if symbol lacks constructor informa…

…tion

This issue is hidden by the fix in the previous commits, so we cannot test
for it directly any more. Applying this patch to
fa38843 exposes the "Constructor not found".

Fixes #2498

bin/varnishtest/tests/v00016.vtc

@@ -102,3 +102,14 @@ varnish v1 -errvcl {Undefined acl foo} {
 		}
 	}
 }
+
+varnish v1 -errvcl {Symbol not found: 'foo'} {
+	backend b { .host = "127.0.0.1"; }
+
+	acl foo {
+	    "127.0.0.1"/32;
+	}
+	sub vcl_init {
+		new bar = foo;
+	}
+}

lib/libvcc/vcc_vmod.c

@@ -291,8 +291,12 @@ vcc_ParseNew(struct vcc *tl)
 
 	ExpectErr(tl, ID);
 	sy2 = VCC_SymbolTok(tl, NULL, tl->t, SYM_OBJECT, 0);
-	if (sy2 == NULL) {
-		VSB_printf(tl->sb, "Symbol not found: ");
+	if (sy2 == NULL || sy2->extra == NULL) {
+		if (sy2 == NULL)
+			p = "Symbol";
+		else if (sy2->extra == NULL)
+			p = "Constructor";
+		VSB_printf(tl->sb, "%s not found: ", p);
 		vcc_ErrToken(tl, tl->t);
 		VSB_printf(tl->sb, " at ");
 		vcc_ErrWhere(tl, tl->t);
@@ -301,6 +305,7 @@ vcc_ParseNew(struct vcc *tl)
 	vcc_NextToken(tl);
 
 	p = sy2->extra;
+	AN(p);
 
 	s_obj = p;
 	p += strlen(p) + 1;