ash: avoid crash when job table is empty

Commit 7b692ddf0 (ash: improved support for jobs built-in) didn't
correctly initialise the pointer to the job table if it was empty.
This resulted in the following crashing:

   sh -c "cat <(echo HelloWorld | rev)"

Fix forkshell_copy() so the job table pointer is NULL if there are
no jobs.

Adds 16 bytes.

(GitHub issue #379)

1 files changed, 8 insertions, 6 deletions

diff --git a/shell/ash.c b/shell/ash.c
index d81d27d25..8b7c6c627 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -17058,12 +17058,14 @@ forkshell_copy(struct forkshell *fs, struct forkshell *new)
                 }
 #endif
 #if JOBS_WIN32
-                new->jobtab = jobtab_copy();
-                SAVE_PTR(new->jobtab, "jobtab", NO_FREE);
-                new->njobs = njobs;
-                if (curjob) {
-                        new->curjob = new->jobtab + (curjob - jobtab);
-                        SAVE_PTR(new->curjob, "curjob", NO_FREE);
+                if (njobs) {
+                        new->jobtab = jobtab_copy();
+                        SAVE_PTR(new->jobtab, "jobtab", NO_FREE);
+                        new->njobs = njobs;
+                        if (curjob) {
+                                new->curjob = new->jobtab + (curjob - jobtab);
+                                SAVE_PTR(new->curjob, "curjob", NO_FREE);
+                        }
                 }
 #endif
         }