TFORM crashes in CleanUpSort when TempSortDir is unavailable

[tueda]

If the sort file can't be created at startup, Terminate(-1) is called (#527):

https://github.com/vermaseren/form/blob/8abec49f876deae1d13ad159519b301f0b140dc5/sources/startup.c#L807-L810

This eventually leads to a call to CleanUpSort(0). But, at startup, B (inside GETIDENTITY) is NULL in

https://github.com/vermaseren/form/blob/8abec49f876deae1d13ad159519b301f0b140dc5/sources/sort.c#L4803-L4808

So, AN causes a null pointer dereference.

This results in a segfault before printing an error message on Windows, giving a test failure.