test-logging: Fix -Werror=maybe-uninitialized warning

test-logging: Fix -Werror=maybe-uninitialized warning

Checking for uninitialized variables raises warning for file path
variables in test_logfile_write and test_logfile_lock functions.

To suppress this warning, initialize varibles to NULL. This is safe
change as result of g_build_filename is stored to them before any usage.

Signed-off-by: Miroslav Rezanina <[email protected]>
Message-Id: <63b0fcedf7dfe799c8210b113e5dccf32414a89d.1579598240[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Reviewed-by: Robert Foley <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>