Guardedalloc: Add extra logging and checks in MEM_freeN()

We don't like when NULL is send to MEM_freeN(), but there was some
differences between lockfree and guarded allocators:

- Lockfree would have silently crash, in both release and debug modes
- Guarded allocator would have printed error message, abort in debug
  but keep working in release build.

This commit makes lockfree allocator behavior to match guarded one.

intern/guardedalloc/intern/mallocn.c

@@ -17,6 +17,7 @@
  *
  * Contributor(s): Brecht Van Lommel
  *                 Campbell Barton
+ *                 Sergey Sharybin
  *
  * ***** END GPL LICENSE BLOCK *****
  */

intern/guardedalloc/intern/mallocn_guarded_impl.c

@@ -22,6 +22,7 @@
  *
  * Contributor(s): Brecht Van Lommel
  *                 Campbell Barton
+ *                 Sergey Sharybin
  *
  * ***** END GPL LICENSE BLOCK *****
  */

intern/guardedalloc/intern/mallocn_lockfree_impl.c

@@ -134,6 +134,14 @@ void MEM_lockfree_freeN(void *vmemh)
 	MemHead *memh = MEMHEAD_FROM_PTR(vmemh);
 	size_t len = MEM_lockfree_allocN_len(vmemh);
 
+	if (vmemh == NULL) {
+		print_error("Attempt to free NULL pointer\n");
+#ifdef WITH_ASSERT_ABORT
+		abort();
+#endif
+		return;
+	}
+
 	atomic_sub_u(&totblock, 1);
 	atomic_sub_z(&mem_in_use, len);