sprintf: handle sized int-ish formats with Inf/Nan

sprintf: handle sized int-ish formats with Inf/Nan

The code path taken when int-ish formats saw an Inf/Nan was to jump to the
floating-point handler, but then that would warn about (valid) size
qualifiers. For example before:

    $ perl -we'printf "[%hi]\n", Inf'
    Invalid conversion in printf: "%hi" at -e line 1.
    Redundant argument in printf at -e line 1.
    [%hi]
    $

After this commit:

    $ perl -we'printf "[%hi]\n", Inf'
    [Inf]
    $

It also makes the code simpler.