Use g_ascii_isalnum() rather than isalnum().

Only *ASCII* alphanumerics are allowed in filterable field names, so use
g_ascii_isalnum() to check for them.  That avoids issues with characters
with the 8th bit set and avoids locale-dependent behavior.

Change-Id: I4e3c4eec907f5e576629229fcf154fcf728b7a2e
Reviewed-on: https://code.wireshark.org/review/4973
Reviewed-by: Guy Harris <guy@alum.mit.edu>
(cherry picked from commit 8ea4df97e052f3fb0af3336990f2bef6d882b258)
Reviewed-on: https://code.wireshark.org/review/4974