Which line$($s$)$ if any might crash when calling these functions in a multi$-$threaded program? lock$($mutex$)$ void iter$\_$next$($iter$\_$t$*$ iter$)\{$

$2.$ if $($iter$)\{$

$3.$ if $($iter$->$curr$)\{$

$4.$ lock$($&mutex$)$;

$5.$ iter$->$curr $=$ iter$->$curr$->$next;

$6.$ unlock$($&mutex$)$;

$7.\}$

$8.\}$

$9.\}$

$10.$ void$*$ iter$\_$get$($iter$\_$t$*$ iter$)\{$

$11.$ if $($iter$)\{$

$12.$ if $($iter$->$curr$)\{$

$13.$ return iter$->$curr$->$obj;

$14.\}$

$15.\}$

$16.$ return NULL;

$17.$