Bug#15869 Cannot shutdown the server - it restarts

 - 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

@@ -329,12 +329,14 @@ int sigwait(sigset_t *setp, int *sigp);		/* Use our implemention */
   we want to make sure that no such flags are set.
 */
 #if defined(HAVE_SIGACTION) && !defined(my_sigset)
-#define my_sigset(A,B) do { struct sigaction s; sigset_t set;              \
+#define my_sigset(A,B) do { struct sigaction s; sigset_t set; int rc;      \
+                            DBUG_ASSERT((A) != 0);                         \
                             sigemptyset(&set);                             \
                             s.sa_handler = (B);                            \
                             s.sa_mask    = set;                            \
                             s.sa_flags   = 0;                              \
-                            sigaction((A), &s, (struct sigaction *) NULL); \
+                            rc= sigaction((A), &s, (struct sigaction *) NULL);\
+                            DBUG_ASSERT(rc == 0);                          \
                           } while (0)
 #elif defined(HAVE_SIGSET) && !defined(my_sigset)
 #define my_sigset(A,B) sigset((A),(B))

sql/mysqld.cc

@@ -884,7 +884,8 @@ static void __cdecl kill_server(int sig_ptr)
     RETURN_FROM_KILL_SERVER;
   kill_in_progress=TRUE;
   abort_loop=1;					// This should be set
-  my_sigset(sig,SIG_IGN);
+  if (sig != 0) // 0 is not a valid signal number
+    my_sigset(sig,SIG_IGN);
   if (sig == MYSQL_KILL_SIGNAL || sig == 0)
     sql_print_information(ER(ER_NORMAL_SHUTDOWN),my_progname);
   else