I think it would make sense to add the check for syscall.Sendto. The other functions (SendtoInet4, SendtoInet6, SendmsgNInet4, SendmsgNInet6) are defined in internal/syscall/unix and are only used internally by Go. As far as I can see all call sites pass a non-nil Sockaddr, so I'm not sure we'd need the check there.
Same as CL 433504 did for the syscall package.
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