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 <librick-openwrt@proton.me>

net/net-snmp/files/snmpd.init

@@ -160,6 +160,7 @@ snmpd_access_HostIP_add() {
 	config_get host_ip "$cfg" HostIP
 	[ -n "$host_ip" ] || return 0
 	config_get ip_mask "$cfg" IPMask
+	[ -n "$ip_mask" ] || return 0
 	config_get mode "$cfg" Mode
 	[ -n "$mode" ] || return 0
 	config_get community "$cfg" CommunityName