Fix access to the bintoasc mapping in the libksba support.

* common/ksba-io-support.c (has_only_base64): Use memchr since calling
strchr on a non-NUL terminated string is undefined behavior.

This patch has been stripped from Colin's original patch because this
is not just about a warning but an actual bug. That bug was
introduced in 2003 by me. - wk

• Signed-off-by: Collin Funk <collin.funk1@gmail.com>