Fix integer type mismatch on 64-bit systems

fprintf wants an int parameter for the field width of a "%.*s" expression, but
the code was passing a size_t instead. This potentially broke systems where
sizeof(size_t) != sizeof(int). And even on systems where it did't break,
it still caused a compiler warning.

Signed-off-by: Jonas Fonseca <fonseca@diku.dk>