improved security by encoding NULL values; double free mitigation on by default; more precise free list corruption detection

test/main-override-static.c

@@ -15,9 +15,9 @@ int main() {
   mi_version();
   
   // detect double frees and heap corruption
-  //double_free1();
-  //double_free2();
-  //corrupt_free();
+  // double_free1();
+  // double_free2();
+  // corrupt_free();
 
   void* p1 = malloc(78);
   void* p2 = malloc(24);