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Problem description: When client loses the connection to the MySQL server or 
if the server gets shutdown after mysql_stmt_prepare() then the next 
mysql_stmt_prepare() will return an error(as expected) but consecutive call 
mysql_stmt_execute(), will crash the client program. 
The expected behavior would be, it should through an error.

Analysis: The mysql_stmt_prepare() interns calls the function end_server() 
and net->vio and net->buff are freed and set to NULL. Then the next call 
mysql_stmt_execute() will interns call net_clear() where we are "net->vio" 
with out validating it.

Fix: we are validating the net->vio, before calling net_clear().