ping: Fix an uninitialized variable

The variable oicmp, which holds the original ("quoted packet") ICMP
packet in a structured way, did not have a copy of the original ICMP
packet obtained from the raw data.

The code was accidentally removed in 20b41303140e. Bring it back.

Reported by: Coverity Scan, cy
Reviewed by: cy
CID: 1506960 (UNINIT)
Fixes: 20b41303140eee4dfb896558fb83600c5f013d39
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39233