libsubprocess: Fix guest watcher cancel race

Fix a race in which the guest eventlog watcher cancels just as the guest
namespace is being deleted.  There is a small chance the user will never
see a ENODATA response.

Fixes #2331

src/modules/job-info/guest_watch.c

@@ -611,6 +611,14 @@ static void guest_namespace_watch_continuation (flux_future_t *f, void *arg)
                 if (main_namespace_watch (gw) < 0)
                     goto error;
             }
+            else if (gw->cancel) {
+                /* Racy scenario - user attempted a cancel right as
+                 * ENOTSUP being sent.  Caller won't get a ENODATA
+                 * response b/c the original watcher is now dead.
+                 */
+                errno = ENODATA;
+                goto error;
+            }
             return;
         }
         else {