Replace one more isprint() with g_ascii_isprint().

Change-Id: I7d49b0d250a5918a7437a524492e834588c6c8cf
Reviewed-on: https://code.wireshark.org/review/4999
Petri-Dish: Jeff Morriss <jeff.morriss.ws@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>