Add null httpReply checks to QHttpThreadDelegate

Add null httpReply checks to QHttpThreadDelegate

If a request is aborted while under load, the abort signal can be
queued in front of a signal emitted from the httpReply.
The abort slot is deleting the httpReply and setting it to null.
So when the queued slot is processed the httpReply is null and
caused an MMU fault.

Removed qWarning from existing null checks, as these are expected
if abort is called with precise timing so that it races with the
reply finishing on the socket.

Task-number: QTBUG-26245
Change-Id: I0a7e0223fda1bc01d117fe8a993c7f6e43fd72ff
Reviewed-by: Jonas Gastal <jgastal@profusion.mobi>
Reviewed-by: Richard J. Moore <rich@kde.org>