mapper: use the utf8_to_fs_charset() result

In map_directory_child_fs(), the result of utf8_to_fs_charset() was
never used, because it passed the unmodified "name" parameter to
pfx_dir().

src/mapper.c

@@ -109,7 +109,7 @@ map_directory_child_fs(const struct directory *directory, const char *name,
 	if (parent_fs == NULL)
 		return NULL;
 
-	utf8_to_fs_charset(buffer, name);
+	name = utf8_to_fs_charset(buffer, name);
 	pfx_dir(buffer, name, strlen(name),
 		parent_fs, strlen(parent_fs));
 	return buffer;