Fix for BUG#2045 "Sending SIGHUP to mysqld crashes it if running with --log-bin".

The constructor of Rotate_log_event used when we are rotating our binlog or
relay log, should not assume that there is a nonzero THD available.
For example, when we are reacting to SIGHUP, the THD is 0.
In fact we don't need to use the THD in this constructor;
we can do like for Stop_log_event, and use the minimal Log_event
constructor.
If we were allowed to put Unix-specific commands in the testsuite,
I'd add a test for this (<sigh>).

sql/log.cc

@@ -843,7 +843,7 @@ void MYSQL_LOG::new_file(bool need_lock)
         We log the whole file name for log file as the user may decide
         to change base names at some point.
       */
-      THD* thd = current_thd;
+      THD* thd = current_thd; /* may be 0 if we are reacting to SIGHUP */
       Rotate_log_event r(thd,new_name+dirname_length(new_name));
       r.set_log_pos(this);
       
@@ -852,7 +852,7 @@ void MYSQL_LOG::new_file(bool need_lock)
         the slave running with log-bin, we set the flag on rotate
         event to prevent infinite log rotation loop
       */
-      if (thd->slave_thread)
+      if (thd && thd->slave_thread)
         r.flags|= LOG_EVENT_FORCED_ROTATE_F;
       r.write(&log_file);
       bytes_written += r.get_event_len();

sql/log_event.cc

@@ -128,7 +128,12 @@ Log_event::Log_event(THD* thd_arg, uint16 flags_arg, bool using_trans)
 	       (thd->options & (OPTION_NOT_AUTOCOMMIT | OPTION_BEGIN)));
 }
 
-
+/*
+  This minimal constructor is for when you are not even sure that there is a
+  valid THD. For example in the server when we are shutting down or flushing
+  logs after receiving a SIGHUP (then we must write a Rotate to the binlog but
+  we have no THD, so we need this minimal constructor).
+*/
 Log_event::Log_event()
   :temp_buf(0), exec_time(0), cached_event_len(0), flags(0), cache_stmt(0),
    thd(0)

sql/log_event.h

@@ -607,7 +607,7 @@ public:
   Rotate_log_event(THD* thd_arg, const char* new_log_ident_arg,
 		   uint ident_len_arg = 0,
 		   ulonglong pos_arg = LOG_EVENT_OFFSET)
-    :Log_event(thd_arg,0,0), new_log_ident(new_log_ident_arg),
+    :Log_event(), new_log_ident(new_log_ident_arg),
     pos(pos_arg),ident_len(ident_len_arg ? ident_len_arg :
 			   (uint) strlen(new_log_ident_arg)), alloced(0)
   {}