make fellow_io_fini() idempotent

during error paths, we might call it multiple times

src/fellow_io_aio.c

@@ -161,6 +161,9 @@ fellow_io_init(int fd, unsigned entries, void *base, size_t len,
 void
 fellow_io_fini(fellow_ioctx_t **ctxp)
 {
+	if (*ctxp == NULL)
+		return;
+
 	unsigned entries = fellow_io_entries(*ctxp);
 
 	struct fellow_io_aio_ctx *ctx;

src/fellow_io_threads.c

@@ -156,6 +156,9 @@ fellow_io_fini(fellow_ioctx_t **ctxp)
 {
 	struct fellow_io_thread *ctx;
 
+	if (*ctxp == NULL)
+		return;
+
 	TAKE_OBJ_NOTNULL(ctx, (void **)ctxp, FELLOW_IO_THREAD_MAGIC);
 
 	struct fellow_io_status results[ctx->free->nbits];

src/fellow_io_uring.c

@@ -320,6 +320,9 @@ fellow_io_fini(fellow_ioctx_t **ctxp)
 {
 	struct fellow_io_uring *ctx;
 
+	if (*ctxp == NULL)
+		return;
+
 	TAKE_OBJ_NOTNULL(ctx, (void **)ctxp, FELLOW_IO_URING_MAGIC);
 
 	struct fellow_io_status results[ctx->entries];