Fix fmtstr for BIO_printf() et al

-   If we have a maximum amount of characters permitted to be printed
    (for example "%.2s", which allows for a maximum of 2 chars), we
    minimize the number of characters from the string to printed to
    that size.
-   If there is space for padding and there is a maximum amount of
    characters to print (for example "%3.2s", which shall give at
    least a 1 space padding), the amount of characters to pad with
    gets added to the maximum so the minimum field size (3 in this
    example) gets filled out.

Reviewed-by: Matt Caswell <matt@openssl.org>