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

(cherry picked from commit 9d1afef1dfd3af626c471debe7f09f41d67c401f)

Change-Id: I9be1aaf1eba8da0f1389832508922d19e4a7ce2f
Reviewed-on: https://code.wireshark.org/review/4779
Reviewed-by: Guy Harris <guy@alum.mit.edu>