Initialize clear annotation variable to false so they aren't cleared by accident

When it was uninitialized, cancelling the dialog could lead to the
sheet's annotations being cleared because the variable wasn't set
to false anywhere.

eeschema/schedit.cpp

@@ -1057,7 +1057,7 @@ void SCH_EDIT_FRAME::OnEditItem( wxCommandEvent& aEvent )
 
     case SCH_SHEET_T:
         {
-        bool doClearAnnotation;
+        bool doClearAnnotation = false;
         bool doRefresh = false;
         // Keep trace of existing sheet paths. EditSheet() can modify this list
         SCH_SHEET_LIST initial_sheetpathList( g_RootSheet );