RFR: JDK-8222079: Don't use memset to initialize fields decode_env constructor in disassembler.cpp

[Roman Kennke]

Recent gcc (I use version 9) complains about using memset to initialize 
fields of decode_env. Let's use proper field initializers instead.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8222079
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8222079/webrev.01/

Can I please get a review?

Thanks, Roman