#95: fix convolution

[yosupo06]

CONV_long_empty/1               170 ns          168 ns      4275723
CONV_long_empty/2               166 ns          165 ns      4142894
CONV_long_empty/4               167 ns          165 ns      4365042
CONV_long_empty/8               163 ns          162 ns      4345478
CONV_long_empty/16              166 ns          165 ns      4294479
CONV_long_empty/32              165 ns          164 ns      4280900
CONV_long_empty/64              246 ns          245 ns      2929422
CONV_long_empty/128             251 ns          250 ns      2787112
CONV_long_empty/256             192 ns          187 ns      3932120
CONV_long_empty/512             203 ns          198 ns      3421260
CONV_long_empty/1024            243 ns          236 ns      2986769
CONV_long_empty/2048            330 ns          328 ns      2129323
CONV_long_empty/4096            598 ns          595 ns      1182992
CONV_long_empty/8192           1661 ns         1646 ns       418113
CONV_long_empty/16384          3947 ns         3932 ns       176950
CONV_long_empty/32768          7546 ns         7500 ns        93090
CONV_long_empty/65536         17308 ns        17223 ns        40800
CONV_long_empty/131072        37341 ns        37135 ns        19057
CONV_long_empty/262144        79746 ns        79110 ns         8956
CONV_long_empty/524288       183291 ns       182162 ns         3583
CONV_long_empty/1048576      673876 ns       668803 ns         1070

->

CONV_long_empty/1              7.78 ns         7.75 ns     86059578
CONV_long_empty/2              7.94 ns         7.88 ns     86518020
CONV_long_empty/4              8.01 ns         7.94 ns     88622178
CONV_long_empty/8              7.97 ns         7.92 ns     88668204
CONV_long_empty/16             7.91 ns         7.87 ns     90345896
CONV_long_empty/32             8.02 ns         7.99 ns     87890012
CONV_long_empty/64             7.98 ns         7.94 ns     88263479
CONV_long_empty/128            8.03 ns         7.98 ns     87291591
CONV_long_empty/256            7.99 ns         7.95 ns     88373796
CONV_long_empty/512            8.16 ns         8.09 ns     88018207
CONV_long_empty/1024           7.87 ns         7.85 ns     87899945
CONV_long_empty/2048           8.20 ns         8.13 ns     86882051
CONV_long_empty/4096           7.97 ns         7.94 ns     89368928
CONV_long_empty/8192           7.93 ns         7.90 ns     89835729
CONV_long_empty/16384          7.93 ns         7.90 ns     89980076
CONV_long_empty/32768          7.90 ns         7.87 ns     84226738
CONV_long_empty/65536          8.02 ns         7.98 ns     87424596
CONV_long_empty/131072         7.96 ns         7.92 ns     88758147
CONV_long_empty/262144         7.95 ns         7.92 ns     88759272
CONV_long_empty/524288         7.87 ns         7.85 ns     85864285
CONV_long_empty/1048576        7.95 ns         7.90 ns     88205645

[yosupo06]

fix #95