Printf: impossible buffer overflow fix

Printf: impossible buffer overflow fix

When printing near the end of the buffer, there was an overflow in two cases:

(1) %c and size is zero
(2) %1N, %1I, %1I4, %1I6 (auto-fill field_width for Net or IP), size is
    more than actual length of the net/ip but less than the auto-filled
    field width.

Manual code examination showed that nothing could have ever triggered
this behavior. All older versions of BIRD, including BIRD 3 development
versions, are totally safe. This exact overflow has been found while
implementing a new feature in later commits.