Allow fuzz builds to detect string overruns

Allow fuzz builds to detect string overruns

If FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION is defined then we don't NUL
terminate ASN1_STRING datatypes. This shouldn't be necessary but we add it
any for safety in normal builds.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: David Benjamin <davidben@google.com>