localed: fix invalid free after shifting pointers using strstrip

localed: fix invalid free after shifting pointers using strstrip

After manually editing /etc/locale.gen, calling localectl set-locale
sometimes fails. When it fails, the systemd journal shows:
systemd-localed: free() / invalid pointer.

It turned out that it only fails if some of the uncommented lines in
/etc/locale.gen have leading spaces, as in:

  * C.UTF-8       <= OK
  * en_US.UTF-8   <= OK
  *  fr_FR.UTF-8  <= NOK

After parsing a line from /etc/locale.gen, we use strstrip() to obtain
the "trimmed" line (without leading or trailing spaces).

However, we store the result of strstrip() in the original pointer
containing the untrimmed line. This pointer is later passed to free
(this is done automatically using _cleanup_free_).

This is a problem because if any leading space is present, the pointer
will essentially be shifted from its original value. This will result in
an invalid free upon cleanup.

The same issue is present in the locale_gen_locale_supported function.

Fixed by storing the result of strstrip() in a different pointer.