ntb_tool: ubuf is too small to hold a human readable 64 bit value

ubuf buffer is too small. It should be 18 if a NULL is not needed,
or 19 to hold the NULL terminator for the full 64-BIT value plus
the 0x prefix.

Submitted by: bret_ketchum@dell.com
Reviewed by: markj mav
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D26893