When auto scaling, and the buffer is bigger than big enough
for the biggest possible number, don't try and calculate
the max value that will fit in the buffer - that calc
will overflow (guaranteed) and is useless, the value
we're formatting cannot possibly be bigger. So simply
use the unscaled value (the raw number).
While here, also avoid returning values that are larger
than the buffer len ... while it would be nice to be able
to find out how big the buffer should be so the data will
fit, the interface doesn't really allow that (the buffer
length passed in controls the scaling - at least when
auto scaling) and the code already does "return -1" when
it detects the buffer length is too small, even before
it works out how much would have been needed. So, rather
than returning a value > len (while truncating the result
to fit in len ... all courtesy of snprintf()) return -1
in this case as well.
Also, allow suffix==NULL (meaning "") - there's no reason
not to, and requiring users to pass in an explicit "" is
not useful.
wiz