Fix bad signed/unsigned comparisons

Either by casting, or switching to a more appropriate type
for the variable.
diff --git a/win/rfb_win32/TsSessions.cxx b/win/rfb_win32/TsSessions.cxx
index efe7564..a4fac2f 100644
--- a/win/rfb_win32/TsSessions.cxx
+++ b/win/rfb_win32/TsSessions.cxx
@@ -48,7 +48,7 @@
     id = 0;
     if (!_ProcessIdToSessionId.isValid())
       return;
-    if (processId == -1)
+    if (processId == (DWORD)-1)
       processId = GetCurrentProcessId();
     if (!(*_ProcessIdToSessionId)(GetCurrentProcessId(), &id))
       throw rdr::SystemException("ProcessIdToSessionId", GetLastError());
@@ -72,7 +72,7 @@
 #ifdef RFB_HAVE_WINSTATION_CONNECT
     if (!_WinStationConnect.isValid())
       throw rdr::Exception("WinSta APIs missing");
-    if (sessionId == -1)
+    if (sessionId == (DWORD)-1)
       sessionId = mySessionId.id;
 
     // Try to reconnect our session to the console