expr2bits needs to handle pointer case

[kroening]

This fixes #4168. The pointer case was not handled.

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[tautschnig]

The test has a further problem (I should have noticed when reviewing...): it needs 's/#include <stddef.h>/#include <stdlib.h>/'. Then CI should be happy.

[kroening]

@tautschnig Yes, but that problem's a bug. I'll add the #include for this test, and will make a new one without.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 0c22c03).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/101218184