asn1/a_int.c: fix "next negative minimum" corner case in c2i_ibuf.
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 <c04971a99e5a9ee80eaab4b1deb37e845b0bd697@openssl.org>
Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl