Memory leak was caused by Security_buffer helper class. Its free() method was implemented to set m_allocated=false after freeing context buffer. Since Security_buffer has no means to set m_allocated back to true this leads to not freeing all subsequent context buffers.

Fixed by:
- making Security_buffer::m_allocated const object (so it can be assigned by constructor only)
- rewriting Security_buffer::free() to not alter value of Security_buffer::m_allocated
- Disallow copying/assignment of Security_buffer class instances to prevent new memory leaks in the future

This patch also fixes minor bugs in Map_cache class.