gkeyfilesettingsbackend: Fix basename handling when group is unset

Fix an effective regression in commit
7781a9cbd2fd0aa84bee0f4eee88470640ff6706, which happens when
`convert_path()` is called with a `key` which contains no slashes. In
that case, the `key` is entirely the `basename`.

Prior to commit 7781a9cb, the code worked through a fluke of `i == -1`
cancelling out with the various additions in the `g_memdup()` call, and
effectively resulting in `g_strdup (key)`.

Spotted by Guido Berhoerster.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

gio/gkeyfilesettingsbackend.c

@@ -188,7 +188,12 @@ convert_path (GKeyfileSettingsBackend  *kfsb,
     }
 
   if (basename)
-    *basename = g_memdup2 (last_slash + 1, key_len - (last_slash - key));
+    {
+      if (last_slash != NULL)
+        *basename = g_memdup2 (last_slash + 1, key_len - (last_slash - key));
+      else
+        *basename = g_strdup (key);
+    }
 
   return TRUE;
 }