os/FileJournal.cc: fix potential use after close cases

Fix checks after _open() calls to make sure to use the
filehandle only if open returns '0'.

Fix for:

CID 1128395 (#1 of 1): Use after close (USE_AFTER_FREE)
 pass_closed_arg: Passing closed handle this->fd as an argument
 to pwrite.

CID 1128396 (#1 of 2): Use after close (USE_AFTER_FREE)
 pass_closed_arg: Passing closed handle this->fd as an argument
 to read_header.

CID 1128397 (#1 of 1): Use after close (USE_AFTER_FREE)
 pass_closed_arg: Passing closed handle this->fd as an argument
 to read_header.

CID 1128398 (#1 of 1): Use after close (USE_AFTER_FREE)
 pass_closed_arg: Passing closed handle this->fd as an argument
 to read_header.

CID 1128400 (#1 of 1): Use after close (USE_AFTER_FREE)
 pass_closed_arg: Passing closed handle this->fd as an argument
 to read_header.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>