Fix bad signed/unsigned comparisons

Either by casting, or switching to a more appropriate type
for the variable.
diff --git a/vncviewer/CConn.cxx b/vncviewer/CConn.cxx
index cf4f31b..5935c17 100644
--- a/vncviewer/CConn.cxx
+++ b/vncviewer/CConn.cxx
@@ -134,7 +134,7 @@
 {
   OptionsDialog::removeCallback(handleOptions);
 
-  for (int i = 0; i < sizeof(decoders)/sizeof(decoders[0]); i++)
+  for (size_t i = 0; i < sizeof(decoders)/sizeof(decoders[0]); i++)
     delete decoders[i];
 
   if (desktop)