fixed get_heap_size function by adding null pointer checks.

main.c

@@ -157,6 +157,9 @@ void free(void* ptr)
 /// @returns Usage in bytes.
 size_t get_heap_size()
 {
+	if (!first || !last)
+		return 0;
+
 	return ((char*)last + last->size) - (char*)first;
 }