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

[Roman Kennke]

This backports:
https://bugs.openjdk.java.net/browse/JDK-8222079

It fixes UB and makes newer compilers happy.

It's based on the jdk11u change:
http://cr.openjdk.java.net/~rkennke/JDK-8222079-jdk11u/webrev.00/

It is a fairly straight backport and for the most part matches the
jdk11u variant, except that in jdk8u, we don't have an _offset field
and ctor argument, but we do have an additional _total_ticks field.

JDK8u webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8222079-jdk8u/webrev.00/

Can I please get a review?

Thanks,
Roman