Fixing Set-to-data shortkey

The underlying functionality makes it possible to switch a certain
memory in Cutter to data (via the shortcut `d`). This functionality is
comparable to other similar products. The functionality already existed
in cutter. Unfortunately it was broken due to two problems:
* The underlying action was not added to a widget - this is now fixed,
  by adding it to the `setToDataMenu` - like other related actions
* If the underlying meta was not prevously data, it simply just ignored
  the action, as the size of the data returned uninitilialized memory.
  This is now fixed. Returning -1 from `CutterCore::sizeofDataMeta`
  indicates that the Meta at the address is currently no data.

Author: amibranch

src/core/Cutter.cpp

@@ -866,7 +866,7 @@ void CutterCore::setToData(RVA addr, int size, int repeat)
 
 int CutterCore::sizeofDataMeta(RVA addr)
 {
-    ut64 size;
+    ut64 size = -1;
     CORE_LOCK();
     rz_meta_get_at(core->analysis, addr, RZ_META_TYPE_DATA, &size);
     return (int)size;

src/menus/DisassemblyContextMenu.cpp

@@ -284,6 +284,7 @@ void DisassemblyContextMenu::addSetToDataMenu()
 
     auto switchAction = new QAction(this);
     initShortcutAction(switchAction, "Disassembly.setToData", SLOT(on_actionSetToData_triggered()));
+    setToDataMenu->addAction(switchAction);
 }
 
 void DisassemblyContextMenu::addEditMenu()
@@ -912,6 +913,10 @@ void DisassemblyContextMenu::on_actionSetAsStringAdvanced_triggered()
 void DisassemblyContextMenu::on_actionSetToData_triggered()
 {
     int size = Core()->sizeofDataMeta(offset);
+    if (size < 0) {
+        setToData(1);
+        return;
+    }
     if (size > 8 || (size && (size & (size - 1)))) {
         return;
     }