The only multiplicators used in scalarproduct_and_madd_*
are -1, 0 and +1. Yet it is of type int and the checkasm
test uses the complete range of int for it, leading to overflows
that don't happen for actual users.

Fix this by using a more reasonable range for mul: Given
that it is used in v1[i] += v3[i] * mul with v1 being
a 16bit integer, it makes no sense to use values for mul
that don't fit into 16bit.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>