clamp_row_est avoid infinite

hi.

/*
* clamp_row_est
* Force a row-count estimate to a sane value.
*/
double
clamp_row_est(double nrows)
{
/*
* Avoid infinite and NaN row estimates. Costs derived from such values
* are going to be useless. Also force the estimate to be at least one
* row, to make explain output look better and to avoid possible
* divide-by-zero when interpolating costs. Make it an integer, too.
*/
if (nrows > MAXIMUM_ROWCOUNT || isnan(nrows))
nrows = MAXIMUM_ROWCOUNT;
else if (nrows <= 1.0)
nrows = 1.0;
else
nrows = rint(nrows);

return nrows;
}

The comments say `Avoid infinite and NaN`
but actually we only avoid NaN.

Do we need to add isinf(nrows)?