audio: Clean up inappropriate and unreachable use of hw_error()

audio_init() should not use hw_error(), because dumping CPU registers
is unhelpful there, and aborting is wrong, because it can be called
called from an audio device's realize() method.

The two uses of hw_error() come from commit 0d9acba8:

* When qemu_new_timer() fails.  It couldn't fail back then, and it
  can't fail now.  Drop the unreachable error handling.

* When no_audio_driver can't be initialized.  It couldn't fail back
  then, and it can't fail now.  Replace the error handling by an
  assertion.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@pond.sub.org>
Reviewed-by: NGerd Hoffmann <kraxel@redhat.com>

audio/audio.c

@@ -1806,9 +1806,6 @@ static void audio_init (void)
     atexit (audio_atexit);
 
     s->ts = timer_new_ns(QEMU_CLOCK_VIRTUAL, audio_timer, s);
-    if (!s->ts) {
-        hw_error("Could not create audio timer\n");
-    }
 
     audio_process_options ("AUDIO", audio_options);
 
@@ -1859,12 +1856,8 @@ static void audio_init (void)
 
     if (!done) {
         done = !audio_driver_init (s, &no_audio_driver);
-        if (!done) {
-            hw_error("Could not initialize audio subsystem\n");
-        }
-        else {
-            dolog ("warning: Using timer based audio emulation\n");
-        }
+        assert(done);
+        dolog("warning: Using timer based audio emulation\n");
     }
 
     if (conf.period.hertz <= 0) {