isprint() -> g_ascii_isprint()

(cherry picked from commit 3157bf6ba5e4896b28316695ecd3a4659631c8c2)

Change-Id: I0e4645b83289a4a12b7d1eaa275a17c411ebbfce
Reviewed-on: https://code.wireshark.org/review/4789
Reviewed-by: Guy Harris <guy@alum.mit.edu>