Field initialize m_hash

valgrind complain (rightly) about use of uninitialized
if Structure ctor throws (eg. duplicate field).
~Field will then try to use m_hash before it has been
initialized.  This don't hurt and the subsequent equality
tests prevent any bad behavior.

src/factory/FieldCreateFactory.cpp

@@ -79,6 +79,7 @@ struct FieldCreate::Helper {
 
 Field::Field(Type type)
     : m_fieldType(type)
+    , m_hash(0)
 {
     REFTRACE_INCREMENT(num_instances);
 }