locking: fix a few small issues uncovered by -Wthread-safety

- rpcwallet: No need to lock twice here
- openssl: Clang doesn't understand selective lock/unlock here. Ignore it.
- CNode: Fix a legitimate (though very unlikely) locking bug.

src/net.cpp

@@ -2033,8 +2033,10 @@ void CNode::EndMessage() UNLOCK_FUNCTION(cs_vSend)
         Fuzz(GetArg("-fuzzmessagestest", 10));
 
     if (ssSend.size() == 0)
+    {
+        LEAVE_CRITICAL_SECTION(cs_vSend);
         return;
-
+    }
     // Set the size
     unsigned int nSize = ssSend.size() - CMessageHeader::HEADER_SIZE;
     WriteLE32((uint8_t*)&ssSend[CMessageHeader::MESSAGE_SIZE_OFFSET], nSize);