builder-manifest: Fix an uninitialised variable warning

[bbhtt]

Fixes 7a710f6

In file included from /usr/include/glib-2.0/glib.h:117,
                 from /usr/include/glib-2.0/glib/gi18n.h:23,
                 from ../src/builder-manifest.c:31:
In function ‘g_autoptr_cleanup_generic_gfree’,
    inlined from ‘_rename_icon’ at ../src/builder-manifest.c:2346:28:
/usr/include/glib-2.0/glib/glib-autocleanups.h:32:3: warning: ‘new_name’ may be used uninitialized [-Wmaybe-uninitialized]
   32 |   g_free (*pp);
      |   ^~~~~~~~~~~~
../src/builder-manifest.c: In function ‘_rename_icon’:
../src/builder-manifest.c:2346:28: note: ‘new_name’ was declared here
 2346 |           g_autofree char *new_name;
      |                            ^~~~~~~~