The function thd_security_context allocates memory on an unprotected MEM_ROOT if the
message length becomes longer than requested and the initial buffer memory needs to
be reallocated.
                  
This patch fixes the design error by copying parts of the reallocated buffer 
to the destination buffer. This works because the destination buffer isn't
owned by the String object and thus isn't freed when a new buffer is allocated.
Any new memory allocated by the String object is reclaimed when the object
is destroyed at the end of the function call.