ssd_int8_vs_int16 is only called from encode_block()
in svq1enc.c; it calls it in stages: At stage 0,
the int16_t array contains the difference of two
uint16_t. At each of the following stages, the
int16_t array is filled by subtracting an int8_t from
the current stage's int16_t array. The maximum stage
is five, so the int16_t are in the range
(-255 - 5 * 127)..(255 + 5 * 128).

This commit modifies the checkasm test to only use
values from this range, fixing (undefined) integer overflow
in the test.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>