A buffer large enough to hold the query _plus_ some additional
data is allocated before parsing is started.   The additional data 
is used by the query cache, and consists of the name of the current 
database and a set of flags.
 
When a packet containing multiple SQL statements is sent to the
server and one of the statements changes the current database
(a "USE <db>" statement), and the name of the new current database 
is longer than of the previous,  there is not enough space in the 
buffer for the new name, and we write out over the buffer boundary.

The fix adds an extra field to store the number of bytes
allocated to the database name in the buffer.  If the current
database name changes, and the new name is longer than the
previous one, we refuse to cache the query.