Fix locking error in spawn implementation

This applies a patch from OpenSUSE. Source:
https://build.opensuse.org/package/view_file/shells/ksh/ksh93-spawnlock.dif

| Wed Oct 12 13:23:14 CEST 2016 - mls@suse.de
|
| - fix locking error in spawn implementation [bnc#988213]
|   new patch: ksh93-spawnlock.dif

Unfortunately the bug report is not currently public:
https://bugzilla.opensuse.org/show_bug.cgi?id=988213
but this one seems sensible enough and is in production use,
so I'll take it on faith.

src/cmd/ksh93/sh/jobs.c

@@ -1947,6 +1947,10 @@ void job_fork(pid_t parent)
 		job_lock();
 		jobfork++;
 		break;
+	case -2:
+		jobfork--;
+		job_unlock();
+		break;
 	case 0:
 		jobfork=0;
 		job_unlock();

src/cmd/ksh93/sh/xec.c

@@ -3631,6 +3631,7 @@ static pid_t sh_ntfork(Shell_t *shp,const Shnode_t *t,char *argv[],int *jobid,in
 #   endif /* !_lib_fork */
 	sh_pushcontext(shp,buffp,SH_JMPCMD);
 	errorpush(&buffp->err,ERROR_SILENT);
+	job_lock();		/* errormsg will unlock */
 	jmpval = sigsetjmp(buffp->buff,0);
 	if(jmpval == 0)
 	{
@@ -3732,14 +3733,15 @@ static pid_t sh_ntfork(Shell_t *shp,const Shnode_t *t,char *argv[],int *jobid,in
 		}
 	fail:
 		if(jobfork && spawnpid<0) 
-			job_fork(0);
+			job_fork(-2);
 		if(spawnpid < 0) switch(errno=shp->path_err)
 		{
 		    case ENOENT:
 			errormsg(SH_DICT,ERROR_system(ERROR_NOENT),e_found+4);
 		    default:
 			errormsg(SH_DICT,ERROR_system(ERROR_NOEXEC),e_exec+4);
 		}
+		job_unlock();
 	}
 	else
 		exitset();