go/importer: make For("gccgo", nil) not panic

Apparently we forgot to test this.

Fixes #15092

Change-Id: I33d4fef0f659dfbdfc1ebf8401e96610c8215592
Reviewed-on: https://go-review.googlesource.com/21860Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/importer/importer.go

@@ -31,7 +31,7 @@ func For(compiler string, lookup Lookup) types.Importer {
 		return make(gcimports)
 
 	case "gccgo":
-		if lookup == nil {
+		if lookup != nil {
 			panic("gccgo importer for custom import path lookup not yet implemented")
 		}