Correct buffer->size at the same time as buffer->capacity when shrinking down a

Correct buffer->size at the same time as buffer->capacity when shrinking down a
buffer that has grown beyond max_capacity.  This fixes a Xenconsoled segfault
that was caused by stomping around above the buffer's allocated region.  It
became possible to set the max_capacity with changeset 7431, and ever
since then we have been exposed to this bug.  It would most easily be triggered
by running a domain without a client attached to the console, so that the
max_capacity was reached more easily.

Closes bug #380.

Signed-off-by: Ewan Mellor <ewan@xensource.com>