asn1/a_int.c: remove code duplicate and optimize branches,

i.e. reduce amount of branches and favour likely ones.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3192)

(cherry picked from commit a3ea6bf0)