Fix possible crash when downloading attachments

Turns out we've been happily deleting network replies from the
QNetworkReply::finished(). That was never a good thing to do, but it did
not use to crash with older Qt. Now it does.

After changing to deleteLater(), there's a window for
already-deregistered replies to generate events, therefore the assert
has to go, too, otherwise Bad Things happen:

 (gdb) bt
 #0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
 #1  0x00007ffff16bdcd2 in __GI_abort () at abort.c:89
 #2  0x00007ffff2400bcb in qt_message_fatal (context=..., message=<synthetic pointer>...) at /var/tmp/portage/dev-qt/qtcore-5.13.9999/work/qtcore-5.13.9999/src/corelib/global/qlogging.cpp:1904
 #3  QMessageLogger::fatal (this=this@entry=0x7fffffffc990, msg=msg@entry=0x7ffff2690b10 "ASSERT: \"%s\" in file %s, line %d") at /var/tmp/portage/dev-qt/qtcore-5.13.9999/work/qtcore-5.13.9999/src/corelib/global/qlogging.cpp:888
 #4

  0x00007ffff23fff7c in qt_assert (assertion=assertion@entry=0x5555558451d7 "reply", file=file@entry=0x555555841a38 "/home/jkt/work/prog/trojita/src/Imap/Network/FileDownloadManager.cpp", line=line@entry=142)
     at /var/tmp/portage/dev-qt/qtcore-5.13.9999/work/qtcore-5.13.9999/src/corelib/global/qglobal.cpp:3247
 #5  0x00005555555da840 in Imap::Network::FileDownloadManager::onPartDataTransfered (this=0x555556a20990)
 #6  0x00007ffff25f1bdf in QtPrivate::QSlotObjectBase::call (a=0x7fffffffcaa0, r=0x555556a20990, this=0x5555569f99c0) at ../../include/QtCore/../../../qtcore-5.13.9999/src/corelib/kernel/qobjectdefs_impl.h:394
 #7  QMetaObject::activate(QObject*, int, int, void**) () at /var/tmp/portage/dev-qt/qtcore-5.13.9999/work/qtcore-5.13.9999/src/corelib/kernel/qobject.cpp:3787
 #8  0x00007ffff25f20b7 in QMetaObject::activate (sender=sender@entry=0x555556a21370, m=m@entry=0x7ffff3f96b00 <QNetworkReply::staticMetaObject>, local_signal_index=local_signal_index@entry=1, argv=argv@entry=0x0)
     at /var/tmp/portage/dev-qt/qtcore-5.13.9999/work/qtcore-5.13.9999/src/corelib/kernel/qobject.cpp:3658
 #9  0x00007ffff3d3cbf3 in QNetworkReply::finished (this=this@entry=0x555556a21370) at .moc/moc_qnetworkreply.cpp:385
 #10 0x0000555555709485 in Imap::Network::MsgPartNetworkReply::slotMyDataChanged() () at /home/jkt/work/prog/trojita/src/Imap/Network/MsgPartNetworkReply.cpp:112

BUG: 417697
Reported-by: Stefan de Konink <stefan@konink.de>
Change-Id: I79f340c5a471430a14474472513d0a055c7238d6