output/alsa: add nullptr check for snd_pcm_name() return value

It is not explicitly documented whether snd_pcm_name() is allowed to
return NULL:
https://www.alsa-project.org/alsa-doc/alsa-lib/group___p_c_m.html#ga5031edc0422df8db1f70af056a12dd77

But apparently this is legal:
0222f45d11/src/pcm/pcm.c (L2761-L2762)

That's ... surprising!

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1645

NEWS

@@ -5,6 +5,8 @@ ver 0.23.10 (not yet released)
   - ffmpeg: fix libfmt 9 compiler warning
 * encoder
   - flac: fix failure when libFLAC is built without Ogg support
+* output
+  - alsa: fix crash bug
 * Windows
   - log to stdout by default, don't require "log_file" setting
 

src/output/plugins/AlsaOutputPlugin.cxx

@@ -812,8 +812,12 @@ AlsaOutput::Open(AudioFormat &audio_format)
 				      fmt::format("Failed to open ALSA device \"{}\"",
 						  GetDevice()).c_str());
 
+	const char *pcm_name = snd_pcm_name(pcm);
+	if (pcm_name == nullptr)
+		pcm_name = "?";
+
 	FmtDebug(alsa_output_domain, "opened {} type={}",
-		 snd_pcm_name(pcm),
+		 pcm_name,
 		 snd_pcm_type_name(snd_pcm_type(pcm)));
 
 #ifdef ENABLE_DSD