Allow width_bucket()'s "operand" input to be NaN.

Allow width_bucket()'s "operand" input to be NaN.

The array-based variant of width_bucket() has always accepted NaN
inputs, treating them as equal but larger than any non-NaN,
as we do in ordinary comparisons.  But up to now, the four-argument
variants threw errors for a NaN operand.  This is inconsistent
and unnecessary, since we can perfectly well regard NaN as falling
after the last bucket.

We do still throw error for NaN or infinity histogram-bound inputs,
since there's no way to compute sensible bucket boundaries.

Arguably this is a bug fix, but given the lack of field complaints
I'm content to fix it in master.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Discussion: http://postgr.es/m/2822872.1750540911@sss.pgh.pa.us