Fix checking of category for printf-style qCDebug

Fixes an issue introduced in b8a38a6737. We forgot
to check for category validness, and printed qCDebug(c, ""...)
unconditionally for compilers not supporting variadic
macros.

Change-Id: I779a97dacd7ea559f0d91473aad2c29e73ea2f89
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>

src/corelib/global/qlogging.cpp

@@ -322,6 +322,8 @@ void QMessageLogger::debug(QMessageLogger::CategoryFunction catFunc,
                            const char *msg, ...) const
 {
     const QLoggingCategory &cat = (*catFunc)();
+    if (!cat.isDebugEnabled())
+        return;
 
     QMessageLogContext ctxt;
     ctxt.copy(context);