WBXML: add a basic sanity check for offset overflow

This is a naive approach allowing to detact that something went wrong,
without the need to replace all proto_tree_add_text() calls as what was
done in master-2.0 branch.

Bug: 12408
Change-Id: Ia14905005e17ae322c2fc639ad5e491fa08b0108
Reviewed-on: https://code.wireshark.org/review/15310
Reviewed-by: Michael Mann <mmann78@netscape.net>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
val_to_valstr(guint32 val, const value_valuestring *vvs)
{
	gint i = 0;

	while (vvs[i].valstrptr) {
		if (vvs[i].value == val)
			return(vvs[i].valstrptr);
		i++;
	}

	return(NULL);
}

val_to_valstr(guint32 val, const value_valuestring *vvs)
{
	gint i = 0;

	while (vvs[i].valstrptr) {
		if (vvs[i].value == val)
			return(vvs[i].valstrptr);
		i++;
	}

	return(NULL);
}
