lib: cbprintf: fix mishandling of precision string output

If a precision flag is included for s formatting that bounds the
maximum output length, so we need to use strnlen rather than strlen to
get the amount of data to emit.  With that flag we can't expect there
to be a terminating NUL following the text to print.

Also fix handling of an empty precision, which should behave as if a
precision of zero was provided.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>