QNetworkReplyImplPrivate - potential nullptr dereference

Found by Coverity, CID 11351. If we check 'if (backend)' at one line,
not clear why we do 'if (bakcend->canResume())' two lines above without any test.
Also, 'delete backend' is noop if backend is nullptr, so ...
even this 'if (backend)' is ... not needed.

Change-Id: I7a46ce04baeb9251debb7b246954911df58880ca
Reviewed-by: Edward Welbourne <edward.welbourne@theqtcompany.com>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>

src/network/access/qnetworkreplyimpl.cpp

@@ -1100,21 +1100,17 @@ bool QNetworkReplyImplPrivate::migrateBackend()
         return true;
 
     // Backend does not support resuming download.
-    if (!backend->canResume())
+    if (backend && !backend->canResume())
         return false;
 
     state = QNetworkReplyPrivate::Reconnecting;
 
-    if (backend) {
-        delete backend;
-        backend = 0;
-    }
-
     cookedHeaders.clear();
     rawHeaders.clear();
 
     preMigrationDownloaded = bytesDownloaded;
 
+    delete backend;
     backend = manager->d_func()->findBackend(operation, request);
 
     if (backend) {