cmake: Tolerate missing HELPSTRING on compiler change

Teach cmake::HandleDeleteCacheVariables to tolerate a missing HELPSTRING
(NULL pointer) when saving cache entries.  In the absence of other bugs
this should not be possible, but avoid the crash just in case.

Source/cmake.cxx

@@ -1231,7 +1231,10 @@ int cmake::HandleDeleteCacheVariables(const char* var)
     if(ci.Find(save.key.c_str()))
       {
       save.type = ci.GetType();
-      save.help = ci.GetProperty("HELPSTRING");
+      if(const char* help = ci.GetProperty("HELPSTRING"))
+        {
+        save.help = help;
+        }
       }
     saved.push_back(save);
     }