canbus: clear interface pointer after free

[Opriego]

Summary

• add a focused regression test for canbus_free
• clear canbus->iface after freeing it

Problem

canbus_free uses the equality operator instead of assignment after freeing
canbus->iface. The expression has no effect, leaving a dangling pointer in the
client structure.

Fix

Replace the ineffective comparison with an assignment to NULL.

Testing

Before the fix:

0%: Checks: 1, Failures: 1, Errors: 0
Assertion 'client.iface == NULL' failed

After the fix:

100%: Checks: 1, Failures: 0, Errors: 0

The existing canbus_shutdown pointer/integer comparison warning is unrelated
and remains unchanged.