[0.4.9][SHELL32] Fix uninitialized variable usage. CORE-15251 (#3986)

This fixes log spam about NM_CUSTOMDRAW not being handled,
e.g. when interacting with the start-menu or filebrowsers menubar.
Less logging implies faster painting here with dbg-builds.

fix picked from commit 0.4.15-dev-3229-g 22e58e68aa

The bug was a regression of 0.4.9-dev-151-g 0aed0fd167 very likely,
The symptom with the logging can neither be observed in unpatched releases/0.4.8 nor in releases/0.4.7

dll/win32/shell32/shellmenu/CMenuToolbars.cpp

@@ -261,6 +261,10 @@ HRESULT CMenuToolbarBase::OnCustomDraw(LPNMTBCUSTOMDRAW cdraw, LRESULT * theResu
         }
         *theResult = TRUE;
         return S_OK;
+
+    default:
+        *theResult = 0L;
+        break;
     }
     return S_OK;
 }