Add some CHECKs to track down the source of a NULL deref in the SSLClientSocketWin code.

net/socket/ssl_client_socket_win.cc

Index: net/socket/ssl_client_socket_win.cc
diff --git a/net/socket/ssl_client_socket_win.cc b/net/socket/ssl_client_socket_win.cc
index f40fad406a740e2ba5a955a9f6857a08d502d931..86412d53fa970a5325375d631377ef4c7e663ea5 100644
--- a/net/socket/ssl_client_socket_win.cc
+++ b/net/socket/ssl_client_socket_win.cc
@@ -543,6 +543,10 @@ int SSLClientSocketWin::Read(IOBuffer* buf, int buf_len,
   }
 
   DCHECK(!user_buf_);
+  // http://crbug.com/16371: We're seeing |buf->data()| return NULL.  See if the
+  // user is passing in an IOBuffer with a NULL |data_|.
+  CHECK(buf);
+  CHECK(buf->data());
   user_buf_ = buf;
   user_buf_len_ = buf_len;