sdexec: fix invalid read

Problem: running t2409-sdexec.t under valgrind turned up this invalid
read:

==3088190== Invalid read of size 8
==3088190==    at 0x83D1C45: json_decref (jansson.h:132)
==3088190==    by 0x83D1C45: list_cb (sdexec.c:809)
==3088190==    by 0x4880407: call_handler (msg_handler.c:345)
==3088190==    by 0x4880799: dispatch_message (msg_handler.c:381)
==3088190==    by 0x4880799: handle_cb (msg_handler.c:482)
==3088190==    by 0x48B4DA2: ev_invoke_pending (ev.c:3770)
==3088190==    by 0x48B884F: ev_run (ev.c:4190)
==3088190==    by 0x48B884F: ev_run (ev.c:4021)
==3088190==    by 0x487F48E: flux_reactor_run (reactor.c:128)
==3088190==    by 0x83D254B: mod_main (sdexec.c:1023)
==3088190==    by 0x11EEC0: module_thread (module.c:185)
==3088190==    by 0x4BACB42: start_thread (pthread_create.c:442)
==3088190==    by 0x4C3DBB3: clone (clone.S:100)

Fix a "o" vs "O" issue in json_pack().

Author: garlick

src/modules/sdexec/sdexec.c

@@ -802,7 +802,7 @@ static void list_cb (flux_t *h,
     }
     if (flux_respond_pack (h,
                            msg,
-                           "{s:i s:o}",
+                           "{s:i s:O}",
                            "rank", ctx->rank,
                            "procs", procs) < 0)
         flux_log_error (h, "error responding to list request");