gfileutils: Correct operator precedence to avoid undefined pointer maths

`base` can be `-1` in some situations, which would lead to pointing
outside an allocation area if the sums were evaluated as `(file_name +
base) + 1` rather than `file_name + (base + 1)`.

I don’t see how this can practically cause an issue, as the arithmetic
is all finished before anything’s dereferenced, but let’s keep to the
letter of the C standard to avoid this coming up in code audits in
future.

Fix suggested by fablhx.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #2077

glib/gfileutils.c

@@ -2397,7 +2397,7 @@ g_path_get_basename (const gchar *file_name)
 
   len = last_nonslash - base;
   retval = g_malloc (len + 1);
-  memcpy (retval, file_name + base + 1, len);
+  memcpy (retval, file_name + (base + 1), len);
   retval [len] = '\0';
 
   return retval;