add MEM_SANITY_CHECK (behaves like MEMP_SANITY_CHECK) and ensure 'ram_end->prev' does not get changed

src/include/lwip/opt.h

@@ -295,6 +295,14 @@
 #define MEMP_SANITY_CHECK               0
 #endif
 
+/**
+ * MEM_SANITY_CHECK==1: run a sanity check after each mem_free() to make
+ * sure that the linked list of heap elements is not corrupted.
+ */
+#if !defined MEM_SANITY_CHECK || defined __DOXYGEN__
+#define MEM_SANITY_CHECK                0
+#endif
+
 /**
  * MEM_USE_POOLS==1: Use an alternative to malloc() by allocating from a set
  * of memory pools of various sizes. When mem_malloc is called, an element of