ui-log: handle parse_commit() errors

If parse_commit() fails, none of the fields in the commit structure will
have been populated so we will dereference NULL when accessing
item->tree.

There isn't much we can do about the error at this point, but if we
return true then we'll try parsing the commit again from print_commit()
and we can report an error to the user at that point.

Coverity-id: 13801
Signed-off-by: John Keeping <john@keeping.me.uk>