Stop using a zero precision specifier with sprintf ("%.0d") to cause

Stop using a zero precision specifier with sprintf ("%.0d") to cause
zero to be generated as the empty string rather than "0". Instead, do
the job by the obvious approach of not calling sprintf at all if the
number is zero. Works around a bug in Emscripten's C library, whose
sprintf doesn't correctly handle that corner case.

[originally from svn r9893]