net-snmp: add early return when HostIP is unset

net-snmp: add early return when HostIP is unset

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

This is a breaking change for configs where HostIP is unset.
Previously, an empty HostIP option would result in a malformed
directive containing a bare /mask 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>