Delete ADSBDemod::m_worker after removing sink from DSP

[ChiefGokhlayeh]

Fixes #915.

As described in the issue, during deconstruction of ADSBDemod a segmentation fault might occur, if DSPDeviceSourceEngine calls sink->stop() after m_worker was deleted.

This PR fixes the issue by deleting m_worker only after m_deviceAPI->removeChannelSink(this) is called. This call would otherwise invoke sink->stop() with a broken m_worker somewhere down the line.

[ChiefGokhlayeh]

I scanned through the other channelrx plugins and didn't find any bugs of the same type.

[f4exb]

It does not hurt and incidentally solves the issue. Moreover now the deletion occurs in reverse order of creation (m_networkManager is created last). But as I said in my comments of #915 in the first place the various workers should take care of their running state before doing the stopping job (same thing for start) because their stopWork method may be called several times legitimately.