gitk: Catch mkdtemp errors

gitk: Catch mkdtemp errors

105b5d3f ("gitk: Use mktemp -d to avoid predictable temporary
directories") introduced a dependency on mkdtemp, which is not
available on Windows.

Use the original temporary directory behavior when mkdtemp fails.
This makes the code use mkdtemp when available and gracefully
fallback to the existing behavior when it is not available.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>