drm/amd/amdgpu: Fix missing null check in atombios_i2c.c

Reported by smatch:
amdgpu_atombios_i2c_process_i2c_ch() error: we previously assumed 'buf' could be null

Reviewed-by: Alex Deucher <[e-mail]>
Signed-off-by: Ernst Sjöstrand <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/atombios_i2c.c

···
 			args.ucRegIndex = buf[0];
 		if (num)
 			num--;
-		if (num)
-			memcpy(&out, &buf[1], num);
+		if (num) {
+			if (buf) {
+				memcpy(&out, &buf[1], num);
+			} else {
+				DRM_ERROR("hw i2c: missing buf with num > 1\n");
+				r = -EINVAL;
+				goto done;
+			}
+		}
 		args.lpI2CDataOut = cpu_to_le16(out);
 	} else {
 		if (num > ATOM_MAX_HW_I2C_READ) {