Reset shmem_exit_inprogress after shmem_exit()

In ad9a2747, shmem_exit_inprogress was
introduced.  But we need to reset it after shmem_exit(), because unlike
the similar proc_exit(), shmem_exit() can also be called for cleanup
when the process will not exit.
Reported-by: Andrew Gierth <andrew@tao11.riddles.org.uk>

src/backend/storage/ipc/ipc.c

@@ -272,6 +272,8 @@ shmem_exit(int code)
 		on_shmem_exit_list[on_shmem_exit_index].function(code,
 														 on_shmem_exit_list[on_shmem_exit_index].arg);
 	on_shmem_exit_index = 0;
+
+	shmem_exit_inprogress = false;
 }
 
 /* ----------------------------------------------------------------