QCoreWlanEngine - null the pointer

QCoreWlanEngine is using a global variable but fails to reset it to nil when deleted,
thus re-creating this WLAN manager we use a dangling pointer (found in
qnetworkconfigurationmanagerqappless). Since our API allows to (re)create object(s) of
these class, the pointer must be set to nil after -release call.
Ideally, of course, this class has to be re-factored.

Change-Id: I08662f55dc6cd2ceb0e0cad2574ee3dee6b8e3fd
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>

src/plugins/bearer/corewlan/qcorewlanengine.mm

@@ -85,8 +85,11 @@ extern "C" { // Otherwise it won't find CWKeychain* symbols at link time
     return self;
 }
 
+static QT_MANGLE_NAMESPACE(QNSListener) *listener = 0;
+
 -(void)dealloc
 {
+    listener = nil;
     [super dealloc];
 }
 
@@ -117,7 +120,6 @@ extern "C" { // Otherwise it won't find CWKeychain* symbols at link time
 }
 @end
 
-static QT_MANGLE_NAMESPACE(QNSListener) *listener = 0;
 
 QT_BEGIN_NAMESPACE