asn1/a_strex.c: fix flags truncation in do_esc_char.

|flags| argument to do_esc_char  was apparently truncated by implicit
cast. [Caught by VC warning subsytem.]

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4721)

(cherry picked from commit 37246310)