value for a precision is negative, act as though precision weren't
specified at all, that is the whole .* part of the format spec should
be ignored.  Our previous coding took it as .0 which is certainly
wrong.  Per report from Kris Jurka and local testing.

Possibly this should be back-patched, but it would be good to get
some more testing first; in any case there are no known cases where
there's really a problem on the backend side.