alsa: Prevent SEGFAULT if initialization failed

When calling from the first time and being unable to get the alsa
lock, dring would behave normally.

However, on the second try, it would crash.

Change-Id: I47c4e68f5111c29f356bde88723a585fe34d1a0d
Signed-off-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>

src/media/audio/alsa/alsalayer.cpp

@@ -183,7 +183,7 @@ bool AlsaLayer::openDevice(snd_pcm_t **pcm, const std::string &dev, snd_pcm_stre
         if (err == -EBUSY) {
             // We're called in audioThread_ context, so if exit is requested
             // force return now
-            if (not audioThread_->isRunning())
+            if ((not audioThread_) or (not audioThread_->isRunning()))
                 return false;
             std::this_thread::sleep_for(std::chrono::milliseconds(100));
         }