cmd/gc: check malloc return value

Check the return value from malloc - do not assume that we were
allocated memory just because we asked for it.

Update #4415.

R=minux.ma, daniel.morsing, remyoudompheng, rsc
CC=golang-dev
https://golang.org/cl/6782100

src/cmd/gc/lex.c

@@ -1165,6 +1165,11 @@ l0:
 	case '[':
 		if(loophack || lstk != nil) {
 			h = malloc(sizeof *h);
+			if(h == nil) {
+				flusherrors();
+				yyerror("out of memory");
+				errorexit();
+			}
 			h->v = loophack;
 			h->next = lstk;
 			lstk = h;

src/cmd/gc/sinit.c

@@ -84,6 +84,11 @@ init1(Node *n, NodeList **out)
 	}
 	n->initorder = InitPending;
 	l = malloc(sizeof *l);
+	if(l == nil) {
+		flusherrors();
+		yyerror("out of memory");
+		errorexit();
+	}
 	l->next = initlist;
 	l->n = n;
 	l->end = nil;