Avoid platform-dependent infinite loop in pg_dump.

If malloc(0) returns NULL, the binary search in findSecLabels() will
probably go into an infinite loop when there are no security labels,
because NULL-1 is greater than NULL after wraparound.

(We've seen this pathology before ... I wonder whether there's a way to
detect the class of bugs automatically?)

Diagnosis and patch by Steve Singer, cosmetic adjustments by me