ICMP: Update the snapend for some nested IP packets

ICMP: Update the snapend for some nested IP packets

Update the snapend because extensions (MPLS, ...) may be present
after the IP packet. In this case the current (outer) packet's
snapend is not what ip_print() needs to decode an IP packet nested
in the middle of an ICMP payload.

This prevents that, in ip_print(), for the nested IP packet, the
remaining length < remaining caplen.

Moreover:
Reduce the scope of a variable.
Fix spaces