lavc/x86/hevc_add_res: Fix overflow in ADD_RES_MMX_4_8

lavc/x86/hevc_add_res: Fix overflow in ADD_RES_MMX_4_8

Fix overflow for coeff -32768 in function ADD_RES_MMX_4_8 with no
performance drop.

./checkasm --test=hevc_add_res --bench

Mainline:
  - hevc_add_res.add_residual [OK]
    hevc_add_res_4x4_8_mmxext: 15.5

Add overflow test case:
  - hevc_add_res.add_residual [FAILED]

After:
  - hevc_add_res.add_residual [OK]
    hevc_add_res_4x4_8_mmxext: 15.0

Signed-off-by: Xu Guangxin <guangxin.xu@intel.com>
Signed-off-by: Linjie Fu <linjie.fu@intel.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>