net-snmp: add early return when IPMask is unset

net-snmp: add early return when IPMask is unset

snmpd_access_HostIP_add would write unintended output to the
config file if the IPMask option is not specified. Add an early
return to prevent writing in that case.

This is a breaking change for configs where IPMask is unset.
Previously, an empty IPMask option would result in a malformed
directive containing a trailing slash with no guaranteed newline,
the exact form of which depended on the values of other options.
Now, no directive is written.

Signed-off-by: Eric McDonald <redacted>