Debugger: Fix sub-register display

Inherit value validity from parent; the 'unknown' default
leads to a wrong [inaccessible] display otherwise.

Change-Id: I6ad23b5c3583c3c8fb5cbaa799d7629ff79328ed
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

src/plugins/debugger/registerhandler.cpp

@@ -340,6 +340,7 @@ QByteArray RegisterValue::toByteArray(int base, RegisterKind kind, int size) con
 RegisterValue RegisterValue::subValue(int size, int index) const
 {
     RegisterValue value;
+    value.known = known;
     switch (size) {
         case 1:
             value.v.u8[0] = v.u8[index];