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>
default_opaque_literal_attr(tvbuff_t *tvb, guint32 offset,
			    const char *token _U_, guint8 codepage _U_, guint32 *length)
{
	guint32 data_len = tvb_get_guintvar(tvb, offset, length);
	char *str = wmem_strdup_printf(wmem_packet_scope(), "(%d bytes of opaque data)", data_len);
	*length += data_len;
	return str;
}

default_opaque_literal_attr(tvbuff_t *tvb, guint32 offset,
			    const char *token _U_, guint8 codepage _U_, guint32 *length)
{
	guint32 data_len = tvb_get_guintvar(tvb, offset, length);
	char *str = wmem_strdup_printf(wmem_packet_scope(), "(%d bytes of opaque data)", data_len);
	*length += data_len;
	return str;
}
