Mapper: _get_music_directory_utf8() may return nullptr

If no music_directory is configured, return nullptr instead of an
empty string.  This fixes a crash when db_file is configured without
music_directory.

NEWS

@@ -1,4 +1,6 @@
 ver 0.18.5 (20??/??/??)
+* configuration
+  - fix crash when db_file is configured without music_directory
 * decoder
   - fluidsynth: auto-detect by default
 * fix ia64, mipsel and other little-endian architectures

src/Mapper.cxx

@@ -133,7 +133,9 @@ void mapper_finish(void)
 const char *
 mapper_get_music_directory_utf8(void)
 {
-	return music_dir_utf8.c_str();
+	return music_dir_utf8.empty()
+		? nullptr
+		: music_dir_utf8.c_str();
 }
 
 const AllocatedPath &

src/Mapper.hxx

@@ -41,7 +41,8 @@ mapper_init(AllocatedPath &&music_dir, AllocatedPath &&playlist_dir);
 void mapper_finish(void);
 
 /**
- * Return the absolute path of the music directory encoded in UTF-8.
+ * Return the absolute path of the music directory encoded in UTF-8 or
+ * nullptr if no music directory was configured.
  */
 gcc_const
 const char *