Fix crash in close_ps() for NaN input coordinates.

The Assert() here seems unreasonably optimistic.  Andreas Seltenreich
found that it could fail with NaNs in the input geometries, and it
seems likely to me that it might fail in corner cases due to roundoff
error, even for ordinary input values.  As a band-aid, make the function
return SQL NULL instead of crashing.

Report: <87d1md1xji.fsf@credativ.de>