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().

Reviewed-by: Marco Pagani <[e-mail]>
Fixes: ccbc1c302115 ("fpga: add an initial KUnit suite for the FPGA Manager")
Signed-off-by: Qasim Ijaz <[e-mail]>
Acked-by: Xu Yilun <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Xu Yilun <[e-mail]>

drivers/fpga/tests/fpga-mgr-test.c

···
 	img_buf = init_test_buffer(test, IMAGE_SIZE);
 
 	sgt = kunit_kzalloc(test, sizeof(*sgt), GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, sgt);
 	ret = sg_alloc_table(sgt, 1, GFP_KERNEL);
 	KUNIT_ASSERT_EQ(test, ret, 0);
 	sg_init_one(sgt->sgl, img_buf, IMAGE_SIZE);