Fix memcheck errors like "Syscall param sendmsg(msg.msg_control) points to uninitialised byte(s)"

[DL6ER]

Hey Simon,

another patch fixing a small memcheck error reported during Pi-hole
testing. The error is caused in send_from() (forward.c) by handing a
partially uninitialized stack variable to sendmsg() when nowild == false.

Full error output (example):

==1052839== Syscall param sendmsg(msg.msg_control) points to
uninitialised byte(s)
==1052839== at 0x4B7199D: __libc_sendmsg (sendmsg.c:28)
==1052839== by 0x4B7199D: sendmsg (sendmsg.c:25)
==1052839== by 0x21EADB: send_from (forward.c:101)
==1052839== by 0x222551: receive_query (forward.c:1988)
==1052839== by 0x20FD6A: check_dns_listeners (dnsmasq.c:1886)
==1052839== by 0x2120EF: main (dnsmasq.c:1278)
==1052839== Location 0x1fff000098is 24bytes inside local var "control_u"
==1052839== declared at forward.c:49, in frame #1of thread 1
==1052839== Uninitialised value was created by a stack allocation
==1052839== at 0x21EA11: send_from (forward.c:38)

Note that the line-numbers are not necessarily 100% accurate. We have
not noticed any abnormal behavior, however, fixing this reported error
is easy enough.

Best,
Dominik

https://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2024q3/017654.html