Added TRASH_FREED_MEMORY compilation option

One can use -DTRASH_FREED_MEMORY to enable TRASH
macros. Useful to do when one suspects that there
is accesses to freed memory.

Extended my_free() to TRASH freed memory

include/my_valgrind.h

@@ -32,7 +32,7 @@
 # define MEM_CHECK_DEFINED(a,len) ((void) 0)
 #endif /* HAVE_VALGRIND_MEMCHECK_H */
 
-#ifndef DBUG_OFF
+#if !defined(DBUG_OFF) || defined(TRASH_FREED_MEMORY)
 #define TRASH_FILL(A,B,C) do { const size_t trash_tmp= (B); memset(A, C, trash_tmp); MEM_UNDEFINED(A, trash_tmp); } while (0)
 #else
 #define TRASH_FILL(A,B,C) do{ const size_t trash_tmp __attribute__((unused)) = (B) ; MEM_CHECK_ADDRESSABLE(A,trash_tmp);MEM_UNDEFINED(A,trash_tmp);} while (0)

mysys/my_malloc.c

@@ -214,6 +214,13 @@ void my_free(void *ptr)
     my_bool old_flags;
     old_size= MALLOC_SIZE_AND_FLAG(ptr, &old_flags);
     update_malloc_size(- (longlong) old_size - MALLOC_PREFIX_SIZE, old_flags);
+#ifndef SAFEMALLOC
+    /*
+      Trash memory if not safemalloc. We don't have to do this if safemalloc
+      is used as safemalloc will also do trashing
+    */
+    TRASH_FREE(ptr, old_size);
+#endif
     sf_free(MALLOC_FIX_POINTER_FOR_FREE(ptr));
   }
   DBUG_VOID_RETURN;