Segfaults in surface test

[Rubo3]

When I run the test surface in release mode, it always segfaults, sometimes with malloc_consolidate(): unaligned fastbin chunk detected, or free(): double free detected in tcache 2. While in debug mode I also get corrupted double-linked list, munmap_chunk(): invalid pointer, free(): invalid pointer, double free or corruption (!prev) or malloc(): unsorted double linked list corrupted.

[jpbruyere]

That's an easy bug to find. Do you want me to correct it, or do I let you find it, that's in tests/surface.c