sdexec: fix memory leak

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

==3040827== 86 (80 direct, 6 indirect) bytes in 1 blocks are definitely lost in loss record 2 of 7
==3040827==    at 0x484DA83: calloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==3040827==    by 0x83D415F: sdexec_channel_create (channel.c:140)
==3040827==    by 0x83D42CC: sdexec_channel_create_input (channel.c:189)
==3040827==    by 0x83D2040: sdproc_create (sdexec.c:511)
==3040827==    by 0x83D2040: exec_cb (sdexec.c:587)
==3040827==    by 0x4880407: call_handler (msg_handler.c:345)
==3040827==    by 0x4880799: dispatch_message (msg_handler.c:381)
==3040827==    by 0x4880799: handle_cb (msg_handler.c:482)
==3040827==    by 0x48B4DA2: ev_invoke_pending (ev.c:3770)
==3040827==    by 0x48B884F: ev_run (ev.c:4190)
==3040827==    by 0x48B884F: ev_run (ev.c:4021)
==3040827==    by 0x487F48E: flux_reactor_run (reactor.c:128)
==3040827==    by 0x83D253B: mod_main (sdexec.c:1022)
==3040827==    by 0x11EEC0: module_thread (module.c:185)
==3040827==    by 0x4BACB42: start_thread (pthread_create.c:442)

Free the input channel when the sdproc is destroyed.

Author: garlick

src/modules/sdexec/sdexec.c

@@ -390,6 +390,7 @@ static void sdproc_destroy (struct sdproc *proc)
 {
     if (proc) {
         int saved_errno = errno;
+        sdexec_channel_destroy (proc->in);
         sdexec_channel_destroy (proc->out);
         sdexec_channel_destroy (proc->err);
         if (proc->f_watch) {