awk: fix corner case in awk_printf

Example where it wasn't working:
    awk 'BEGIN { printf "qwe %s rty %c uio\n", "a", 0, "c" }'
- the NUL printing in %c caused premature stop of printing.

function                                             old     new   delta
awk_printf                                           593     596      +3

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed