QLibrary: use qEnvironmentVariableIntValue()

It doesn't allocate memory, so cannot throw and is a lot faster
than qgetenv().

Change-Id: I8d440619edfbd90045564e1f92676f1e1f87e136
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/plugin/qlibrary.cpp

@@ -1131,7 +1131,7 @@ bool qt_debug_component()
 {
     static int debug_env = -1;
     if (debug_env == -1)
-       debug_env = QT_PREPEND_NAMESPACE(qgetenv)("QT_DEBUG_PLUGINS").toInt();
+       debug_env = QT_PREPEND_NAMESPACE(qEnvironmentVariableIntValue)("QT_DEBUG_PLUGINS");
 
     return debug_env != 0;
 }