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 <alexander.deucher@amd.com>
Signed-off-by: Ernst Sjöstrand <ernstp@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -65,8 +65,15 @@ static int amdgpu_atombios_i2c_process_i2c_ch(struct amdgpu_i2c_chan *chan,
 			args.ucRegIndex = buf[0];
 		if (num)
 			num--;
-		if (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) {