Error earlier if we cannot find the probe

We don't have forward declarations in VCL.  OK'd by phk@.
Fixes #1837.

bin/varnishtest/tests/r01837.vtc

+varnishtest "Test VCC errors out if probe is used before it is defined"
+
+varnish v1 -errvcl "Probe p not found" {
+	backend b {
+		.host = "127.0.0.1";
+		.probe = p;
+	}
+	probe p {
+	}
+}

bin/varnishtest/tests/v00034.vtc

@@ -20,7 +20,7 @@ varnish v1 -errvcl {Backend s1 redefined} {
 }
 
 varnish v1 -errvcl {Probe p1 redefined} {
-	backend s1 { .host = "127.0.0.1"; .probe = p1;}
 	probe p1 { }
 	probe p1 { }
+	backend s1 { .host = "127.0.0.1"; .probe = p1;}
 }

lib/libvcc/vcc_backend.c

@@ -384,6 +384,12 @@ vcc_ParseHostDef(struct vcc *tl, const struct token *t_be, const char *vgcname)
 			Fb(tl, 0, "\t.probe = &%s,\n", p);
 			ERRCHK(tl);
 		} else if (vcc_IdIs(t_field, "probe") && tl->t->tok == ID) {
+			if (VCC_FindSymbol(tl, tl->t, SYM_PROBE) == NULL) {
+				VSB_printf(tl->sb, "Probe %.*s not found\n",
+				    PF(tl->t));
+				vcc_ErrWhere(tl, tl->t);
+				return;
+			}
 			Fb(tl, 0, "\t.probe = &vgc_probe_%.*s,\n", PF(tl->t));
 			vcc_AddRef(tl, tl->t, SYM_PROBE);
 			vcc_NextToken(tl);