isdigit -> g_ascii_isdigit to handle 8-bit bytes correctly.

(cherry picked from commit 9d1afef1dfd3af626c471debe7f09f41d67c401f)

Change-Id: Icee86ee3efd31df183ff259d605ab0ca0295ae88
Reviewed-on: https://code.wireshark.org/review/4780
Reviewed-by: Guy Harris <guy@alum.mit.edu>