Currently the code in defvert_remove_group() does a malloc and memcpy where a realloc could be used instead. Since the allocation is always smaller, it can be resized in-place and never need to call an internal memcpy (if MEM_reallocN()'s implementation was to do this in the future). Also added an assert to make sure that the existing array is never null, instead of seeming optional as is. If it is null, the [re]allocated elements would contain garbage.