Fix ftbfs on Fedora 34

error: ordered comparison of pointer with integer zero (‘snd_mixer_t*’ {aka ‘_snd_mixer*’} and ‘int’)

Signed-off-by: François Andriot <francois.andriot@free.fr>
(cherry picked from commit 97da3a13f2)

tderadio3/plugins/alsa-sound/alsa-sound.cpp

@@ -908,7 +908,7 @@ static int mixer_dummy_callback(snd_mixer_t *, unsigned int /*mask*/, snd_mixer_
 bool AlsaSoundDevice::openMixerDevice(snd_mixer_t *&mixer_handle, int card, bool reopen, TQTimer *timer, int timer_latency)
 {
     if (reopen) {
-        if (mixer_handle >= 0)
+        if (mixer_handle != NULL)
             closeMixerDevice(mixer_handle, card, SoundStreamID::InvalidID, NULL, /* force = */ true, timer);
         else
             return true;