an assertion in a debug build.

The reason is that the C API doesn't support multiple result sets for prepared
statements and attempting to execute a stored routine which returns multiple result
sets sometimes lead to a network error. The network error sets the diagnostic area
prematurely which later leads to the assert when an attempt is made to set a second
server state.

This patch fixes the issue by changing the scope of the error code returned by
sp_instr_stmt::execute() to include any error which happened during the execution.
To assure that Diagnostic_area::is_sent really mean that the message was sent all
network related functions are checked for return status.

libmysqld/lib_sql.cc:
  * Changed prototype to return success/failure status on net_send_error_packet(),
    net_send_ok(), net_send_eof(), write_eof_packet().
mysql-test/r/sp_notembedded.result:
  * Added test case for bug 44521
mysql-test/t/sp_notembedded.test:
  * Added test case for bug 44521
sql/protocol.cc:
  * Changed prototype to return success/failure status on net_send_error_packet(),
    net_send_ok(), net_send_eof(), write_eof_packet().
sql/protocol.h:
  * Changed prototype to return success/failure status on net_send_error_packet(),
    net_send_ok(), net_send_eof(), write_eof_packet().
sql/sp_head.cc:
  * Changed prototype to return success/failure status on net_send_error_packet(),
    net_send_ok(), net_send_eof(), write_eof_packet().