thermal: int340x: Check for NULL after calling kmemdup()

As the potential failure of the allocation, kmemdup() may return NULL.

Then, 'bin_attr_data_vault.private' will be NULL, but
'bin_attr_data_vault.size' is not 0, which is not consistent.

Therefore, it is better to check the return value of kmemdup() to
avoid the confusion.

Fixes: 0ba13c76 ("thermal/int340x_thermal: Export GDDV")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>