Assignment operator does not check for self-assignment

Defect 99631 (STATIC_C)
Checker SELF_ASSIGN (subcategory none)
File: /src/mongo/db/s/active_migrations_registry.cpp
Function mongo::ScopedRegisterMigration::operator =(mongo::ScopedRegisterMigration&&)
/src/mongo/db/s/active_migrations_registry.cpp, line: 98
No protection against the object assigning to itself.

        _registry = other._registry;