Fix uninitialized-pointer crash and double-free in ModuleFrn init failure paths

[MarkRose]

• audio_fifo was never set in the ModuleFrn constructor, only assigned
  later in initialize(). If Module::initialize() fails early (e.g. a
  version mismatch), ModuleFrn::initialize() returns before any of the
  audio pipeline members (audio_fifo, audio_splitter, audio_valve) are
  assigned, leaving them as garbage/null. The destructor then
  unconditionally calls audio_fifo->unregisterSource(),
  audio_splitter->removeSink(), and audio_valve->unregisterSink() on
  those pointers, crashing the module. Fixed by initializing audio_fifo
  to null in the constructor and guarding all three dereferences in
  moduleCleanup() with null checks.

• When qso->initOk() returns false (any required FRN config variable is
  missing), initialize() deletes qso but leaves the member pointing at
  the freed object. The module is then destroyed, and moduleCleanup()
  uses the dangling qso pointer (audio_splitter->removeSink(qso)) and
  deletes it a second time. Fixed by nulling qso immediately after the
  delete on this failure path.

Co-Authored-By: Claude Opus 4.8 [email protected]

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This PR also adds a unit test (ModuleFrnTest.cpp). It is auto-discovered and executed by the CTest suite proposed in #762 once that is merged; without that suite present the test file is inert and does not affect the build.