Check for EOF in ASCII conversions.

The C standard defines EOF as:

    ... an integer constant expression, with type int and a negative value...

This means a conforming implemenetation could define this as a one of the
printable characters.  This won't be a problem for ASCII.

A specific test case has been added for EOF.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4240)