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Check correctness of indexes in designators

This is an error and can generate a segmentation fault if
it is not controlled.

--- a/cc1/init.c

+++ b/cc1/init.c

@@ -31,6 +31,7 @@

 {

 	TINT npos;

 	Node *np;

+	Type *tp = ip->type;

 	if (ip->type->op != ARY)

 		errorp("array index in non-array initializer");

@@ -37,6 +38,10 @@

 	next();

 	np = iconstexpr();

 	npos = np->sym->u.i;

+	if (npos < 0 || tp->defined && npos >= tp->n.elem) {

+		errorp("array index in initializer exceeds array bounds");

+		npos = 0;

+	}

 	freetree(np);

 	expect(']');

 	return npos;