Fix CoreAudio driver crash when starting input with uninitialized device

Initialize input device if null before starting audio input to prevent
crashes when input_unit is not yet initialized.

drivers/coreaudio/audio_driver_coreaudio.mm

@@ -520,6 +520,8 @@ void AudioDriverCoreAudio::finish_input_device() {
 }
 
 Error AudioDriverCoreAudio::input_start() {
+	ERR_FAIL_NULL_V(input_unit, FAILED);
+
 	input_buffer_init(capture_buffer_frames);
 
 	OSStatus result = AudioOutputUnitStart(input_unit);