PackWriter open: clean up fd if fdopen fails
PackWriter open: clean up fd if fdopen fails

Signed-off-by: Rob Browning <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>
Tested-by: Rob Browning <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>

gevaerts/bup,ToxicFrog/bup,gevaerts/bup,ToxicFrog/bup,ToxicFrog/bup,gevaerts/bup,gevaerts/bup,gevaerts/bup,ToxicFrog/bup,ToxicFrog/bup