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Allow assignation of 0 to pointers

The standard allows that a 0 constant is also a valid NULL pointer constant,
so it can be assigned and compared against pointers.

--- a/cc1/expr.c

+++ b/cc1/expr.c

@@ -413,11 +413,11 @@

 		break;

 	case PTR:

 		switch (newtp->op) {

-		case ENUM:  /* TODO: allow p = 0 */

+		case ENUM:

 		case INT:

 		case VOID:

 			if (!iscast)

-				return NULL;;

+				return NULL;

 			break;

 		case PTR:

 			if (iscast ||

@@ -627,8 +627,12 @@

 	lp = eval(lp);

 	rp = eval(rp);

-	if ((rp = convert(rp, lp->type, 0)) == NULL)

-		error("incompatible types when assigning");

+	if (BTYPE(rp) == INT && BTYPE(lp) == PTR &&

+	    rp->constant && SYMICMP(rp->sym, 0)) {

+		rp = node(OCAST, pvoidtype, rp, NULL);

+	} else if ((rp = convert(rp, lp->type, 0)) == NULL) {

+		errorp("incompatible types when assigning");

+	}

 	return node(op, lp->type, lp, rp);

 }