gvc gvprintpointflist: rewrite for clarity

gvc gvprintpointflist: rewrite for clarity

The unusual structure of the loop in this function made it appear as if it was
intending to cope with e.g. an `n` of 0 but still print the first element of
`p`. Surveying the callers of `gvprintpointflist`, we can see it is never used
this way. So we can write it simpler and more readably.