Fix a potential denial of service in the :mod:`imaplib` module. When connecting to a malicious server, it could cause an arbitrary amount of memory to be allocated. On many systems this is harmless as unused virtual memory is only a mapping, but if this hit a virtual address size limit it could lead to a :exc:`MemoryError` or other process crash. On unusual systems or builds where all allocated memory is touched and backed by actual ram or storage it could've consumed resources doing so until similarly crashing.