mapper: allocate playlist path from heap

Don't pass a static buffer to map_spl_utf8_to_fs().

src/mapper.h

@@ -93,9 +93,10 @@ map_spl_path(void);
 
 /**
  * Maps a playlist name (without the ".m3u" suffix) to a file system
- * path.
+ * path.  The return value is allocated on the heap and must be freed
+ * with g_free().
  */
-const char *
-map_spl_utf8_to_fs(const char *name, char *buffer);
+char *
+map_spl_utf8_to_fs(const char *name);
 
 #endif