Zero size array causes crash in AsyncUDPSocket on llvm15

Summary:
AsyncUDPSocket test cases are crashing when running under llvm15. During
debugging it seems that the issue is the fact that the code tries to allocated 0
size array. Changing the code to prevent such allocation.

This is not very clean why to fix, but I am not sure if there is better one.
Please let me know if you have any suggestions.

Sample crash:
```
$ buck test //folly/io/async/test:async_udp_socket_test
...
stdout:
Note: Google Test filter = AsyncUDPSocketTest.TestDetachAttach
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from AsyncUDPSocketTest
[ RUN      ] AsyncUDPSocketTest.TestDetachAttach

stderr:
fbcode/folly/io/async/AsyncUDPSocket.cpp:699:10: runtime error: variable length array bound evaluates to non-positive value 0
    #0 0x7f4d8ed93704 in folly::AsyncUDPSocket::writev(folly::SocketAddress const&, iovec const*, unsigned long, int) fbcode/folly/io/async/AsyncUDPSocket.cpp:698
    #1 0x7f4d8ed9081f in folly::AsyncUDPSocket::writeGSO(folly::SocketAddress const&, std::unique_ptr<folly::IOBuf, std::default_delete<folly::IOBuf>> const&, int) fbcode/folly/io/async/AsyncUDPSocket.cpp:528
    #2 0x7f4d8ed900b2 in folly::AsyncUDPSocket::write(folly::SocketAddress const&, std::unique_ptr<folly::IOBuf, std::default_delete<folly::IOBuf>> const&) fbcode/folly/io/async/AsyncUDPSocket.cpp:660
    #3 0x350a05 in AsyncUDPSocketTest_TestDetachAttach_Test::TestBody() fbcode/folly/io/async/test/AsyncUDPSocketTest.cpp:914
    #4 0x7f4d90dd1ad5 in testing::Test::Run() /home/engshare/third-party2/googletest/1.11.0/src/googletest/googletest/src/gtest.cc:2682:50
    #5 0x7f4d90dd1ad5 in testing::Test::Run() /home/engshare/third-party2/googletest/1.11.0/src/googletest/googletest/src/gtest.cc:2672:6
    #6 0x7f4d90dd1c64 in testing::TestInfo::Run() /home/engshare/third-party2/googletest/1.11.0/src/googletest/googletest/src/gtest.cc:2861:14
    #7 0x7f4d90dd1c64 in testing::TestInfo::Run() /home/engshare/third-party2/googletest/1.11.0/src/googletest/googletest/src/gtest.cc:2833:6
    #8 0x7f4d90dd2321 in testing::TestSuite::Run() /home/engshare/third-party2/googletest/1.11.0/src/googletest/googletest/src/gtest.cc:3015:31
    #9 0x7f4d90dd2321 in testing::TestSuite::Run() /home/engshare/third-party2/googletest/1.11.0/src/googletest/googletest/src/gtest.cc:2993:6
    #10 0x7f4d90dd2b1e in testing::internal::UnitTestImpl::RunAllTests() /home/engshare/third-party2/googletest/1.11.0/src/googletest/googletest/src/gtest.cc:5855:47
    #11 0x7f4d90dd1d87 in bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) /home/engshare/third-party2/googletest/1.11.0/src/googletest/googletest/src/gtest.cc:2665:29
    #12 0x7f4d90dd1d87 in testing::UnitTest::Run() /home/engshare/third-party2/googletest/1.11.0/src/googletest/googletest/src/gtest.cc:5438:55
    #13 0x7f4d90d5c990 in RUN_ALL_TESTS() fbcode/third-party-buck/platform010/build/googletest/include/gtest/gtest.h:2490
    #14 0x7f4d90d5c618 in main fbcode/common/gtest/LightMain.cpp:20
    #15 0x7f4d8ea2c656 in __libc_start_call_main /home/engshare/third-party2/glibc/2.34/src/glibc-2.34/csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #16 0x7f4d8ea2c717 in __libc_start_main@GLIBC_2.2.5 /home/engshare/third-party2/glibc/2.34/src/glibc-2.34/csu/../csu/libc-start.c:409:3
    #17 0x33ea60 in _start /home/engshare/third-party2/glibc/2.34/src/glibc-2.34/csu/../sysdeps/x86_64/start.S:116
...
```

Reviewed By: russoue, dmm-fb

Differential Revision: D43858875

fbshipit-source-id: 93749bab17027b6dfc0dbc01b6c183e501a5494c

folly/io/async/AsyncUDPSocket.cpp

@@ -693,6 +693,11 @@ ssize_t AsyncUDPSocket::writev(
       cmsgs_->size() * (CMSG_SPACE(sizeof(int)) / CMSG_SPACE(sizeof(uint16_t)));
 
   if (nontrivialCmsgs_.empty() && controlBufSize <= kSmallSizeMax) {
+    // Avoid allocating 0 length array. Doing so leads to exceptions
+    if (controlBufSize == 0) {
+      return writevImpl(&msg, gso);
+    }
+
     // suppress "warning: variable length array 'control' is used [-Wvla]"
     FOLLY_PUSH_WARNING
     FOLLY_GNU_DISABLE_WARNING("-Wvla")