Fix "wuffs gen" null pointer dereference
diff --git a/cmd/wuffs/gen.go b/cmd/wuffs/gen.go
index c982f09..836a42c 100644
--- a/cmd/wuffs/gen.go
+++ b/cmd/wuffs/gen.go
@@ -84,12 +84,14 @@
 	h := genHelper{
 		wuffsRoot:   wuffsRoot,
 		langs:       langs,
-		ccompilers:  *ccompilersFlag,
 		cformatter:  *cformatterFlag,
 		genlinenum:  *genlinenumFlag,
 		skipgen:     genlib && *skipgenFlag,
 		skipgendeps: *skipgendepsFlag,
 	}
+	if genlib {
+		h.ccompilers = *ccompilersFlag
+	}
 
 	for _, arg := range args {
 		recursive := strings.HasSuffix(arg, "/...")