fpga: fix potential null pointer deref in fpga_mgr_test_img_load_sgt()

In the Linux kernel, the following vulnerability has been resolved:

fpga: fix potential null pointer deref in fpga_mgr_test_img_load_sgt()

fpga_mgr_test_img_load_sgt() allocates memory for sgt using

kunit_kzalloc() however it does not check if the allocation failed.

It then passes sgt to sg_alloc_table(), which passes it to

__sg_alloc_table(). This function calls memset() on sgt in an attempt to

zero it out. If the allocation fails then sgt will be NULL and the

memset will trigger a NULL pointer dereference.

Fix this by checking the allocation with KUNIT_ASSERT_NOT_ERR_OR_NULL().