RFR(S): JDK-8211026 jdb should not allow to assign null to value type

[Frederic Parain]

Please review this small fix preventing assigning null to a local variable
with an inline type.

CR:
https://bugs.openjdk.java.net/browse/JDK-8211026

Webrev:
http://cr.openjdk.java.net/~fparain/jdb_null_assignment/webrev.00/index.html

The case of preventing assigning null to an inline field has already been
fixed in a previous patch.

Thank you,

Fred