asn1/a_int.c: fix "next negative minimum" corner case in c2i_ibuf.

"Next" refers to negative minimum "next" to one presentable by given
number of bytes. For example, -128 is negative minimum presentable by
one byte, and -256 is "next" one.

Thanks to Kazuki Yamaguchi for report, GH#3339

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>