Fix a crash when 'kill %%' and 'kill %+' are run (#35)

Ksh was trying to use the 'pw' variable as a valid pointer even
when it was NULL. This is fixed by doing the error check for
'pw' before doing anything else in 'job_kill'.

This bugfix is from Red Hat:
https://git.centos.org/rpms/ksh/blob/44e0a643a93492b1f6beebbf6ffcfd453d9ab8f2/f/SOURCES/ksh-20130214-fixkill.patch

Fixes #34

NEWS

@@ -11,6 +11,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 	cmd=stop; $cmd $!
   will now work. See 'stop --man' and 'suspend --man' for more information.
 
+- Fixed a bug that caused the kill and stop commands to segfault when given
+  a non-existent job.
+
 2020-06-20:
 
 - Fixed a bug that caused setting the following variables as readonly in

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <[email protected]>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-06-20"
+#define SH_RELEASE	"93u+m 2020-06-22"

src/cmd/ksh93/sh/jobs.c

@@ -1111,6 +1111,8 @@ static struct process *job_bystring(register char *ajob)
 
 int job_kill(register struct process *pw,register int sig)
 {
+	if(!pw)
+		goto error;
 	Shell_t	*shp = pw->p_shp;
 	register pid_t pid;
 	register int r;
@@ -1122,8 +1124,6 @@ int job_kill(register struct process *pw,register int sig)
 #endif	/* SIGTSTP */
 	job_lock();
 	errno = ECHILD;
-	if(pw==0)
-		goto error;
 	pid = pw->p_pid;
 #if SHOPT_COSHELL
 	if(pw->p_cojob)

src/cmd/ksh93/tests/signal.sh

@@ -460,5 +460,12 @@ then	[[ $actual == *Killed*Killed* ]] && msg='ksh killed itself' || msg='unexpec
 fi
 let "${actual##*$'\n'} > 128" || err_exit "child process signal did not cause exit status > 128"
 
+# ======
+# Killing a non-existent job shouldn't cause a segfault. Note that `2> /dev/null` has no effect when
+# there is a segfault.
+$SHELL -c 'kill %% 2> /dev/null'; [[ $? == 1 ]] || err_exit $'`kill` doesn\'t handle a non-existent job correctly when passed \'%%\''
+$SHELL -c 'kill %+ 2> /dev/null'; [[ $? == 1 ]] || err_exit $'`kill` doesn\'t handle a non-existent job correctly when passed \'%+\''
+$SHELL -c 'kill %- 2> /dev/null'; [[ $? == 1 ]] || err_exit $'`kill` doesn\'t handle a non-existent job correctly when passed \'%-\''
+
 # ======
 exit $((Errors<125?Errors:125))