Fix extension length counter in client hello parsing.

Summary: Orvid noticed this was always throwing on a properly formated client hello, since the sig algs extension length isn't subtracted from the counter. This doesn't actually affect any behavior (except possibly a slight perf hit), but is pretty clowny.

Reviewed By: anirudhvr

Differential Revision: D3722887

fbshipit-source-id: 579993caac96da24fb567ab977b09fca519750a0

folly/io/async/AsyncSSLSocket.cpp

@@ -1651,6 +1651,7 @@ void AsyncSSLSocket::clientHelloParsingCallback(int written,
         extensionsLength -= 2;
         uint16_t extensionDataLength = cursor.readBE<uint16_t>();
         extensionsLength -= 2;
+        extensionsLength -= extensionDataLength;
 
         if (extensionType == ssl::TLSExtension::SIGNATURE_ALGORITHMS) {
           cursor.skip(2);
@@ -1666,7 +1667,6 @@ void AsyncSSLSocket::clientHelloParsingCallback(int written,
           }
         } else {
           cursor.skip(extensionDataLength);
-          extensionsLength -= extensionDataLength;
         }
       }
     }