folly/io/async/AsyncSocket.cpp: avoid shadowing warning for "rv"

folly/io/async/AsyncSocket.cpp: avoid shadowing warning for "rv"

Summary:
Enabling the proposed-for-upstream gcc -Wshadow-local option exposes this shadowing:
```
folly/io/async/AsyncSocket.cpp: In member function 'virtual void folly::AsyncSocket::connect(folly::AsyncSocket::ConnectCallback*, const folly::SocketAddress&, int, const OptionMap&, const folly::SocketAddress&)':
folly/io/async/AsyncSocket.cpp:421:11: error: declaration of 'rv' shadows a previous local [-Werror=shadow-compatible-local]
folly/io/async/AsyncSocket.cpp:364:9: error: shadowed declaration is here [-Werror=shadow-compatible-local]
```

Reviewed By: ngoyal, knekritz

Differential Revision: D3963873

fbshipit-source-id: 85b3ab1fc3b200b81615a00d66a183cc9c792a90