broaden 0 check in gvprintdouble

broaden 0 check in gvprintdouble

The 0 path in this function is, as the comment says, to avoid printing confusing
numbers like -0.00. However, the remainder of the function prints the number to
2 decimal places. So actually any number that *rounds* to -0.00 is going to come
out this way. To avoid this, we can expand the cases where we take an early
exit. This is also a minor performance speed up in these cases, as the 0 path is
faster than the common path.