xar: fix fflags_text leak in file_free

xar: fix fflags_text leak in file_free

file_free() releases pathname, symlink, uname, gname, and hardlink but
omits fflags_text. When a XAR archive describes a file with <flags> or
<ext2> children (e.g. <SystemNoUnlink/>, <Compress/>), xml_parse_file_flags
/ xml_parse_file_ext2 populate xar_file->fflags_text via archive_strcat,
which heap-allocates. The buffer leaks on every file_free().

Reproduces with ASan+LSan via the bundled bsdtar:
  bsdtar -tvf <xar-with-flags>
=> Direct leak of N bytes ... archive_strcat ... xml_parse_file_flags

Same shape as commit 6767cbe3 ("Free XAR xattr fstype metadata"), which
fixed the analogous miss in xattr_free().

Existing release of fflags_text in archive_string_free is a no-op when
the field was never populated (.s == NULL, free(NULL) is safe), so the
patch is harmless on the non-flags path.

Resolves #3058.