Add overflow test for hevc_add_res when int16_t coeff = -32768.

The result of C is good, while ASM is not.

To verify:
    make fate-checkasm-hevc_add_res
    ffmpeg/tests/checkasm/checkasm --test=hevc_add_res

./checkasm --test=hevc_add_res
checkasm: using random seed 679391863
MMXEXT:
    hevc_add_res_4x4_8_mmxext (hevc_add_res.c:69)
  - hevc_add_res.add_residual [FAILED]
SSE2:
    hevc_add_res_8x8_8_sse2 (hevc_add_res.c:69)
    hevc_add_res_16x16_8_sse2 (hevc_add_res.c:69)
    hevc_add_res_32x32_8_sse2 (hevc_add_res.c:69)
  - hevc_add_res.add_residual [FAILED]
AVX:
    hevc_add_res_8x8_8_avx (hevc_add_res.c:69)
    hevc_add_res_16x16_8_avx (hevc_add_res.c:69)
    hevc_add_res_32x32_8_avx (hevc_add_res.c:69)
  - hevc_add_res.add_residual [FAILED]
AVX2:
    hevc_add_res_32x32_8_avx2 (hevc_add_res.c:69)
  - hevc_add_res.add_residual [FAILED]
checkasm: 8 of 14 tests have failed
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>