initialize msghdr in a compatible manner

msghdr stuct from socket.h is not same between musl and glibc
where musl claims to be more posix  compliant where as glibc seems
to fill whats needed for linux sizewise and chooses long enough types
which maybe questionable, therefore constructing a structure with explicit
constructor is not going to work correctly for musl and glibc at same time

see
https://git.musl-libc.org/cgit/musl/commit/arch/x86_64/bits/socket.h?id=7168790763cdeb794df52be6e3b39fbb021c5a64

This fix initialized the struct to 0 first and then sets the struct elements
by name, so we dont have to hard code the positions of elements when initializing
structure

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>

--- a/src/3rdparty/chromium/net/socket/udp_socket_posix.cc
+++ b/src/3rdparty/chromium/net/socket/udp_socket_posix.cc
@@ -1193,8 +1193,12 @@ SendResult UDPSocketPosixSender::Interna
   for (auto& buffer : buffers)
     msg_iov->push_back({const_cast<char*>(buffer->data()), buffer->length()});
   msgvec->reserve(buffers.size());
-  for (size_t j = 0; j < buffers.size(); j++)
-    msgvec->push_back({{nullptr, 0, &msg_iov[j], 1, nullptr, 0, 0}, 0});
+  for (size_t j = 0; j < buffers.size(); j++) {
+    struct msghdr m = {0};
+    m.msg_iov = &msg_iov[j];
+    m.msg_iovlen = 1;
+    msgvec->push_back({m, 0});
+  }
   int result = HANDLE_EINTR(Sendmmsg(fd, &msgvec[0], buffers.size(), 0));
   SendResult send_result(0, 0, std::move(buffers));
   if (result < 0) {