aio: table lookup: verify ctx pointer

Another shortcoming of the table lookup patch was revealed where the pointer
was not being tested before being dereferenced.  Verify this to avoid the
NULL pointer dereference.
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>

fs/aio.c

@@ -812,7 +812,7 @@ static struct kioctx *lookup_ioctx(unsigned long ctx_id)
 		goto out;
 
 	ctx = table->table[id];
-	if (ctx->user_id == ctx_id) {
+	if (ctx && ctx->user_id == ctx_id) {
 		percpu_ref_get(&ctx->users);
 		ret = ctx;
 	}