Improve coding of gistchoose and gistRelocateBuildBuffersOnSplit.

This is mostly cosmetic, but it does eliminate a speculative portability
issue.  The previous coding ignored the fact that sum_grow could easily
overflow (in fact, it could be summing multiple IEEE float infinities).
On a platform where that didn't guarantee to produce a positive result,
the code would misbehave.  In any case, it was less than readable.