net: ReadMsgUDP/WriteMsgUDP fails with WSAEFAULT on Windows when oob is empty but non-nil [1.26 backport]

[gopherbot]

@database64128 requested issue #77875 to be considered for backport to the next 1.26 minor release.

@gopherbot please backport to Go 1.26. The regression was introduced in CL 685418. CL 750420 is a two-line fix with a regression test.