When re-setting (SET GLOBAL debug='') the GLOBAL debug settings the 
server was not freeing the data elements from the top (initial) frame 
before setting them to 0 without freeing the underlying memory. As these 
are global settings there's a chance that something is there already.
Fixed by :
1. making sure the allocated data are cleaned up before re-setting them
while parsing a debug string
2. making sure the stuff allocated in the global settings is freed on 
shutdown.