gc: fix crash when using -u

R=ken2
CC=golang-dev
https://golang.org/cl/4278056

src/cmd/gc/typecheck.c

@@ -1311,7 +1311,7 @@ ret:
 
 	// TODO(rsc): should not need to check importpkg,
 	// but reflect mentions unsafe.Pointer.
-	if(safemode && !incannedimport && !importpkg && t->etype == TUNSAFEPTR)
+	if(safemode && !incannedimport && !importpkg && t && t->etype == TUNSAFEPTR)
 		yyerror("cannot use unsafe.Pointer");
 
 	evconst(n);