QNetworkProxyFactory::systemProxyForQuery crash on Mac during PAC detection

Hi.

We recently switched to Qt 5.8.0 and encountered a bug that looks like a replay – or at least relative – of https://bugreports.qt.io/browse/QTBUG-36787 (EDIT: no, it's unrelated)

Happens on Mac OS Sierra 10.12.3, but not on Mac OS 10.9.

When the User has set „Auto Proxy Discovery“ in his System settings (see screenshot), a QT app will crash with the following dump:

{monospaced}
Thread 27 Crashed:: Background Job Thread
0 QtNetwork 0x0000000106b16535 0x106a73000 + 668981
1 com.apple.CFNetwork 0x00007fffca5238a8 PAC::PACClient::invokeClientCallback() + 102
2 com.apple.CoreFoundation 0x00007fffcb251981 _CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION_ + 17
3 com.apple.CoreFoundation 0x00007fffcb232a7d __CFRunLoopDoSources0 + 557
4 com.apple.CoreFoundation 0x00007fffcb231f76 __CFRunLoopRun + 934
5 com.apple.CoreFoundation 0x00007fffcb231974 CFRunLoopRunSpecific + 420
6 QtNetwork 0x0000000106b159b5 0x106a73000 + 666037
7 QtNetwork 0x0000000106b16791 QNetworkProxyFactory::systemProxyForQuery(QNetworkProxyQuery const&) + 17
8 QtNetwork 0x0000000106b0c9d8 0x106a73000 + 629208
9 QtNetwork 0x0000000106b0e922 QNetworkProxyFactory::proxyForQuery(QNetworkProxyQuery const&) + 50
10 QtNetwork 0x0000000106aa364e 0x106a73000 + 198222
11 QtNetwork 0x0000000106ac1079 0x106a73000 + 319609
12 QtNetwork 0x0000000106ac6013 0x106a73000 + 339987
13 QtNetwork 0x0000000106abdb1e 0x106a73000 + 305950
14 QtNetwork 0x0000000106abd7cb 0x106a73000 + 305099
15 QtNetwork 0x0000000106aa1fc5 NetworkAccessManager::createRequest(QNetworkAccessManager::Operation,QNetworkRequest const&, QIODevice*) + 1653
16 libCWNetworking.dylib 0x00000001062cd0ab CWNetworkAccessManager::createRequest(QNetworkAccessManager::Operation,QNetworkRequest const&, QIODevice*) + 1131{monospaced}

To reproduce, it's sufficient to use the same code as given in the aforementioned Bug.

#include <QtCore/QUrl>
#include <QtNetwork/QNetworkProxyFactory>
main(int argc, char* argv[])

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