IB/uverbs: Fix an potential error pointer dereference

smatch reports the warning below:
drivers/infiniband/core/uverbs_std_types_counters.c:110
ib_uverbs_handler_UVERBS_METHOD_COUNTERS_READ() error: 'uattr'
dereferencing possible ERR_PTR()

The return value of uattr maybe ERR_PTR(-ENOENT), fix this by checking
the value of uattr before using it.

Fixes: ebb6796b ("IB/uverbs: Add read counters support")
Signed-off-by: Xiang Yang <xiangyang3@huawei.com>
Link: https://lore.kernel.org/r/20230804022525.1916766-1-xiangyang3@huawei.comSigned-off-by: Leon Romanovsky <leon@kernel.org>

drivers/infiniband/core/uverbs_std_types_counters.c

@@ -107,6 +107,8 @@ static int UVERBS_HANDLER(UVERBS_METHOD_COUNTERS_READ)(
 		return ret;
 
 	uattr = uverbs_attr_get(attrs, UVERBS_ATTR_READ_COUNTERS_BUFF);
+	if (IS_ERR(uattr))
+		return PTR_ERR(uattr);
 	read_attr.ncounters = uattr->ptr_attr.len / sizeof(u64);
 	read_attr.counters_buff = uverbs_zalloc(
 		attrs, array_size(read_attr.ncounters, sizeof(u64)));