When spread targets have a percent value of zero it's possible for them to
return -Inf scoring because of a float divide by zero. This is very hard for
operators to debug because the string "-Inf" is returned in the API and that
breaks the presentation of debugging data.
Most scoring iterators are bracketed to -1/+1, but spread iterators do not so
that they can handle greatly unbalanced scoring so we can't simply return a -1
score without generating a score that might be greater than the negative scores
set by other spread targets. Instead, track the lowest-seen spread boost and use
that as the spread boost for any cases where we'd divide by zero.
Fixes: #8863
