ncp2222: don't overflow buffer constructing string

Fixes stack-smashing vuln.

Somehow there was already an XXX in the code about this, but nobody realized at
the time it was worth fixing... really?

Bug: 10552
Change-Id: I849068bd6c45146339444f295b72430d3c6e08f2
Reviewed-on: https://code.wireshark.org/review/4653
Reviewed-by: Evan Huus <eapache@gmail.com>