Possible fix for crash caused by dangling pointers

Happened quite rarely, likely caused by dereferencing one of the
QNetworkReply pointers when already deleted through
QObject::deleteLater()
Setting them to 0 after issuing deleteLater() should fix this.

Likely superseeded by Ralf Engels dynamicplaylist changes anyway.

Backtrace:
(this=0x2f9f390) at
../../../../src/services/lastfm/biases/LastFmBias.cpp:231
(this=0x2f9f390, _c=QMetaObject::InvokeMetaMethod, _id=1,
_a=0x7fffe81f1780) at ./LastFmBias.moc:90