hashmap: add an explicit assert() for detecting when objects migrated between threads

When clients don't follow protocol and use the same object from
different threads, then we previously would silently corrupt memory.
With this assert we'll fail with an assert(). This doesn't fix anything
but certainly makes mis-uses easier to detect and debug.

Triggered by https://bugzilla.redhat.com/show_bug.cgi?id=1609349