drm/admgpu: fix dereferencing null pointer context

When user space sets an invalid ta type, the pointer context will be empty.
So it need to check the pointer context before using it
Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Suggested-by: Tim Huang <Tim.Huang@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_psp_ta.c

@@ -334,7 +334,7 @@ static ssize_t ta_if_invoke_debugfs_write(struct file *fp, const char *buf, size
 
 	set_ta_context_funcs(psp, ta_type, &context);
 
-	if (!context->initialized) {
+	if (!context || !context->initialized) {
 		dev_err(adev->dev, "TA is not initialized\n");
 		ret = -EINVAL;
 		goto err_free_shared_buf;