Convert a couple remaining isalpha() and isprint() calls that were left after
the merges to this branch.

This gets it compiling cleanly again.

Change-Id: I7273c5bb684656b26a98cbcb4e279bc0499c812c
Reviewed-on: https://code.wireshark.org/review/4986
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>