Fix crash if no OSS mixer support

Protect MixerOSS:: routines from trying to access uninitialized tables, as
happens when no OSS mixer support is present.  Specifically, if openMixer()
or initMask() error out for whatever reason, then InputSourceNbr() and other
routines will crash becasue num_rec and num_out never get set to zero.

BugLink: https://bugs.launchpad.net/ubuntu/+source/fldigi/+bug/1058298

src/soundcard/mixer.cxx

@@ -105,6 +105,7 @@ void MixerOSS::closeMixer()
 
 void MixerOSS::initValues()
 {
+	if (mixer_fd == -1) return;
 	int devnbr;
 
 	inpsrc0 = GetCurrentInputSource();
@@ -129,6 +130,7 @@ void MixerOSS::initValues()
 
 void MixerOSS::restoreValues()
 {
+	if (mixer_fd == -1) return;
 	int devnbr;
 	devnbr = InputSourceNbr("Line");
 	SetCurrentInputSource(devnbr);
@@ -230,6 +232,7 @@ int MixerOSS::initMask()
 // returns value between 0.0 and 1.0
 double MixerOSS::ChannelVolume(int channel)
 {
+	if (mixer_fd == -1) return 0.0;
 	int vol;
 	int stereo;
 
@@ -282,22 +285,26 @@ void MixerOSS::PCMVolume(double volume )
 
 int MixerOSS::NumOutputVolumes()
 {
+	if (mixer_fd == -1) return 0;
 	return num_out;
 }
 
 const char *MixerOSS::OutputVolumeName( int i )
 {
+	if (mixer_fd == -1) return NULL;
 	const char *labels[] = SOUND_DEVICE_LABELS;
 	return labels[outs[i]];
 }
 
 double MixerOSS::OutputVolume( int i )
 {
+	if (mixer_fd == -1) return 0.0;
 	return ChannelVolume(outs[i]);
 }
 
 void MixerOSS::OutputVolume( int i, double volume )
 {
+	if (mixer_fd == -1) return;
 	int vol = (int)((volume * 100.0) + 0.5);
 	vol = (vol | (vol<<8));
 	ioctl(mixer_fd, MIXER_WRITE(outs[i]), &vol);
@@ -305,17 +312,20 @@ void MixerOSS::OutputVolume( int i, double volume )
 
 int MixerOSS::GetNumInputSources()
 {
+	if (mixer_fd == -1) return 0;
 	return num_rec;
 }
 
 const char *MixerOSS::GetInputSourceName( int i)
 {
+	if (mixer_fd == -1) return NULL;
 	const char *labels[] = SOUND_DEVICE_LABELS;
 	return labels[recs[i]];
 }
 
 int MixerOSS::InputSourceNbr(const char *source)
 {
+	if (mixer_fd == -1) return -1;
 	const char *labels[] = SOUND_DEVICE_LABELS;
 	char lbl[80];
 	int len;
@@ -363,6 +373,7 @@ double MixerOSS::InputVolume()
 
 void MixerOSS::InputVolume( double volume )
 {
+	if (mixer_fd == -1) return;
 	int vol;
 	vol = (int)((volume * 100.0) + 0.5);
 	vol = (vol | (vol<<8));