Eliminating some compiler warnings.

sql/sql_class.h:
  Adding default return clause.

sql/sql_class.h

@@ -1001,6 +1001,7 @@ show_system_thread(enum_thread_type thread)
 {
 #define RETURN_NAME_AS_STRING(NAME) case (NAME): return #NAME
   switch (thread) {
+    static char buf[64];
     RETURN_NAME_AS_STRING(NON_SYSTEM_THREAD);
     RETURN_NAME_AS_STRING(SYSTEM_THREAD_DELAYED_INSERT);
     RETURN_NAME_AS_STRING(SYSTEM_THREAD_SLAVE_IO);
@@ -1008,6 +1009,9 @@ show_system_thread(enum_thread_type thread)
     RETURN_NAME_AS_STRING(SYSTEM_THREAD_NDBCLUSTER_BINLOG);
     RETURN_NAME_AS_STRING(SYSTEM_THREAD_EVENT_SCHEDULER);
     RETURN_NAME_AS_STRING(SYSTEM_THREAD_EVENT_WORKER);
+  default:
+    sprintf(buf, "<UNKNOWN SYSTEM THREAD: %d>", thread);
+    return buf;
   }
 #undef RETURN_NAME_AS_STRING
 }