Fix crash on "bg" of non-job-controlled job

fish -c 'sleep 1 & bg %1' is supposed to fail because the job is not
under job control.

When we try to print the failure, we accidentally still
hold a borrow of the job list.  This blows up because we use
"builtin_print_help_error()" to print the failure message; that
function runs "job_reap()" which wants an exclusive borrow of the
job list. Let's drop our job list earlier.

(cherry picked from commit 26116b477e)

src/builtins/bg.rs

@@ -17,12 +17,13 @@ fn send_to_bg(
             let err = {
                 let job = &jobs[job_pos];
                 wgettext_fmt!(
-                "%ls: Can't put job %s, '%ls' to background because it is not under job control\n",
-                cmd,
-                job.job_id().to_wstring(),
-                job.command()
-            )
+                    "%ls: Can't put job %s, '%ls' to background because it is not under job control\n",
+                    cmd,
+                    job.job_id().to_wstring(),
+                    job.command()
+                )
             };
+            drop(jobs);
             builtin_print_help_error(parser, streams, cmd, &err);
             return STATUS_CMD_ERROR;
         }

tests/checks/job-control-noninteractive.fish

@@ -18,3 +18,6 @@ or echo "pgroups were the same, job control did not work"
 $fish -c 'status job-control full ; $fth report_foreground' &
 wait
 #CHECKERR: background
+
+$fish -c 'sleep .2 & bg %1'
+#CHECKERR: bg: Can't put job 1, 'sleep .2 &' to background because it is not under job control