- A segfault occured when the function 'kill_server' called 
   'my_sigset' with signal number 0. 'my_sigset' is a macro which
   uses 'sigaction' to install the signal handler with an invalid
   signal number will on most platforms return EINVAL but yields
   a segfauilt on IRIX 6.5
 - The server crash was detected by mysqld_safe and it was restarted although
   a shutdown was requested. 
 - Semantics of kill_server(0) is not known, leaving it intact


include/my_pthread.h:
  Check return value from sigaction with a DBUG_ASSERT
  Also DBUG_ASSERT if signal number 0 is passed
sql/mysqld.cc:
  Don't call my_sigset if signo is 0