Fix a few memory leaks and race conditions

[srcejon]

This PR fixes a few memory leaks and race conditions noticed while investigating #2315.

In FreeDV:

• Fix buffer overflow by using freedv_get_n_max_speech_samples instead of freedv_get_n_speech_samples
• Fix memory leak for m_speechOut and m_modIn
• Call stop() before delete, to avoid "QThread: Destroyed while thread is still running"

In mainwindow.cpp/dspengine.cpp:

• Fix memory leak by deleting m_deviceSourceEngine after it is last used.
• Fix race condition by connecting to thread before exit is called.

We still have a problem causing the crash in #2315 though - will add a further comment there.

[sonarcloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud