libsdexec: fix memory leak

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

==3090348== 168 (32 direct, 136 indirect) bytes in 1 blocks are definitely lost in loss record 2 of 7
==3090348==    at 0x4848899: malloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==3090348==    by 0x83E9592: idset_create (idset.c:42)
==3090348==    by 0x83E9F6C: idset_ndecode (idset_decode.c:121)
==3090348==    by 0x83D4A19: sdexec_parse_bitmap (parse.c:58)
==3090348==    by 0x83D2C18: prop_add (start.c:249)
==3090348==    by 0x83D2C18: prop_create (start.c:313)
==3090348==    by 0x83D32A0: sdexec_start_transient_unit (start.c:345)
==3090348==    by 0x83D21A3: exec_cb (sdexec.c:607)
==3090348==    by 0x4880407: call_handler (msg_handler.c:345)
==3090348==    by 0x4880799: dispatch_message (msg_handler.c:381)
==3090348==    by 0x4880799: handle_cb (msg_handler.c:482)
==3090348==    by 0x48B4DA2: ev_invoke_pending (ev.c:3770)
==3090348==    by 0x48B884F: ev_run (ev.c:4190)
==3090348==    by 0x48B884F: ev_run (ev.c:4021)
==3090348==    by 0x487F48E: flux_reactor_run (reactor.c:128)

Fix leak in sdexec_parse_bitmap().

Author: garlick

src/common/libsdexec/parse.c

@@ -71,10 +71,10 @@ int sdexec_parse_bitmap (const char *s, uint8_t **bp, size_t *sp)
             BITMAP_BYTE (bitmap, id) |= BITMAP_BIT (id);
             id = idset_next (ids, id);
         }
-        idset_destroy (ids);
     }
     *bp = bitmap;
     *sp = BITMAP_NBYTES (nbits);
+    idset_destroy (ids);
     return 0;
 }