folly: async: fix SIOF in test

folly: async: fix SIOF in test

Summary: avoid sillyness:

```
$ _build/opt/folly/io/async/test/async_test --gtest_list_tests
ASAN:SIGSEGV
=================================================================
==3245135==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000010 (pc 0x000000583444 sp 0x7fff17ba0c40 bp 0x7fff17ba0c80 T0)
    #0 0x583443 in std::_Rb_tree<int, std::pair<int const, folly::SSLContext::SSLLockType>, std::_Select1st<std::pair<int const, folly::SSLContext::SSLLockType> >, std::less<int>, std::allocator<std::pair<int const, folly::SSLContext::SSLLockType> > >::operator=(std::_Rb_tree<int, std::pair<int const, folly::SSLContext::SSLLockType>, std::_Select1st<std::pair<int const, folly::SSLContext::SSLLockType> >, std::less<int>, std::allocator<std::pair<int const, folly::SSLContext::SSLLockType> > > const&) third-party2/libgcc/4.9.x/gcc-4.9-glibc-2.20-fb/024dbc3/include/c++/4.9.x/bits/stl_tree.h:1138
    #1 0x583443 in std::map<int, folly::SSLContext::SSLLockType, std::less<int>, std::allocator<std::pair<int const, folly::SSLContext::SSLLockType> > >::operator=(std::map<int, folly::SSLContext::SSLLockType, std::less<int>, std::allocator<std::pair<int const, folly::SSLContext::SSLLockType> > > const&) third-party2/libgcc/4.9.x/gcc-4.9-glibc-2.20-fb/024dbc3/include/c++/4.9.x/bits/stl_map.h:295
    #2 0x583443 in folly::SSLContext::setSSLLockTypes(std::map<int, folly::SSLContext::SSLLockType, std::less<int>, std::allocator<std::pair<int const, folly::SSLContext::SSLLockType> > >) folly/io/async/SSLContext.cpp:682
    #3 0x40e028 in Initializer folly/io/async/test/AsyncSSLSocketTest2.cpp:143
    #4 0x40e028 in __static_initialization_and_destruction_0 folly/io/async/test/AsyncSSLSocketTest2.cpp:146
    #5 0x40e028 in _GLOBAL__sub_I__ZN5folly47AsyncSSLSocketTest2_AttachDetachSSLContext_Test10test_info_E folly/io/async/test/AsyncSSLSocketTest2.cpp:147
    #6 0x66cf2e in __libc_csu_init /home/engshare/third-party2/glibc/2.20/src/glibc-2.20/csu/elf-init.c:88
    #7 0x7f7145600084 in __libc_start_main (/usr/local/fbcode/gcc-4.9-glibc-2.20-fb/lib/libc.so.6+0x20084)
    #8 0x410be5 (/data/users/lucian/fbcode2/_build/opt/folly/io/async/test/async_test+0x410be5)

AddressSanitizer can not provide additional info.
AAAAAAASUMMARY: AddressSanitizer: SEGV third-party2/libgcc/4.9.x/gcc-4.9-glibc-2.20-fb/024dbc3/include/c++/4.9.x/bits/stl_tree.h:1138 std::_Rb_tree<int, std::pair<int const, folly::SSLContext::SSLLockType>, std::_Select1st<std::pair<int const, folly::SSLContext::SSLLockType> >, std::less<int>, std::allocator<std::pair<int const, folly::SSLContext::SSLLockType> > >::operator=(std::_Rb_tree<int, std::pair<int const, folly::SSLContext::SSLLockType>, std::_Select1st<std::pair<int const, folly::SSLContext::SSLLockType> >, std::less<int>, std::allocator<std::pair<int const, folly::SSLContext::SSLLockType> > > const&)
==3245135==ABORTING
```

Reviewed By: @philippv

Differential Revision: D2440796