Internals: Fix cppcheck warnings; add VL_DANGLING. No functional change.

include/verilatedos.h

@@ -95,6 +95,12 @@
 // This is not necessarily the same as #UL, depending on what the IData typedef is.
 #define VL_UL(c) ((IData)(c##UL))	///< Add appropriate suffix to 32-bit constant
 
+#ifdef VL_CPPCHECK
+# define VL_DANGLING(v)
+#else
+# define VL_DANGLING(v) do { (v) = NULL; } while(0)	///< After e.g. delete, set variable to NULL to indicate must not use later
+#endif
+
 //=========================================================================
 // C++-2011