ossl_asn1.c: fix undefined behavior

* ext/openssl/ossl_asn1.c (ossl_asn1_initialize): SYMID on a value
  other than Symbol is an undefined behavior.  fix up r31699.
  [ruby-core:62142] [Bug #9771]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45701 b2dd03c8-39d4-4d8f-98ff-823fe69b080e