Fix compiler warnings in Cube Extension

Hi,

My compiler is complaining about cube_coord() and cube_coord_llur() not
returning a value on all code paths. On looking at this I noticed that
this is happening due the pattern used is not quite aligned with other
code, as normally we do:

if (<error condition>)
ereport(<some error>);

do stuff;
PG_RETURN_....(value);

I've rearranged the code to be aligned more with what's normal. In passing
I also adding some missing and removed some unneeded parenthesis, and also
adjusted some white space.

--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services