fix nullptr-dereference in SocketAddress

Summary: When `getaddrinfo` fails, `SocketAddress` attempts to describe the error and throw an exception with that description. It includes the `gai_strerror`, which is a C-string, without checking whether the result is nullptr. On linux it is not `nullptr` but you never know.

Reviewed By: knekritz

Differential Revision: D70713343

fbshipit-source-id: 8d6c3710e51610a1ca43886f668941f5e4713054

Author: yfeldblum

folly/SocketAddress.cpp

@@ -115,7 +115,7 @@ struct GetAddrInfoError {
   }
 #else
   const char* error;
-  const char* str() const { return error; }
+  const char* str() const { return error ? error : "Unknown error"; }
   explicit GetAddrInfoError(int errorCode) : error(gai_strerror(errorCode)) {}
 #endif
 };