QNetworkProxyFactory::systemProxyForQuery crash on Mac during PAC detection

QNetworkProxyFactory::systemProxyForQuery() can crash on Mac if proxy auto-configuration using PAC is enabled and the URL contains spaces (or other non-URL chars).

0   com.apple.CoreFoundation      	0x00007fff96c1bf45 CFURLCopyScheme + 37
1   com.apple.CoreFoundation      	0x00007fff96c1ab20 CFURLCreateDataAndPropertiesFromResource + 64
2   QtNetwork                     	0x0000000105e0b16f macQueryInternal(QNetworkProxyQuery const&) + 991
3   QtNetwork                     	0x0000000105e0c452 QNetworkProxyFactory::systemProxyForQuery(QNetworkProxyQuery const&) 

The problem is that the URL passed to CFURLCreateDataAndPropertiesFromResource() may be null.

Steps to reproduce:

1. Go to System Preferences -> Network
2. Select the active network connection in the left-hand connection list
3. Click the 'Advanced' button in the right-hand pane
4. Select the 'Proxies' tab.
5. Select the 'Automatic Proxy Configuration' option in the left-hand proxy type list
6. In the URL field on the right, enter the following URL, note the spaces: "http://www.foobar.com/ invalid url"
7. Click OK
8. Click Apply
9. Build and run the following test case:

#include <QtCore/QUrl>
#include <QtNetwork/QNetworkProxyFactory>

int main(int,char**)
{
	QNetworkProxyFactory::systemProxyForQuery(QNetworkProxyQuery(QUrl("http://qt-project.org")));
	return 0;
}