BUG#55263: assert in check_binlog_magic

The procedure for setting up a fake binary log, by changing the
relay log files manually, is run twice when we issue mtr with
--repeat=2. However, when setting it up for the second time,
neither the sql thread is reset nor the server is restarted. This
means that previous stale relay log IO_CACHE metadata - from 
first run - is left around. As a consequence, when the test is 
run for the second time, the IO_CACHE for the relay log has 
position in file different than zero, triggering the assertion.
            
We fix this by deploying a call to my_b_seek before calling
check_binlog_magic in next_event. This prevents the server 
from asserting, in the cases that the SQL thread was reads
from a hot log (relay.NNNNN), then is stopped, then is restarted 
from a previous cold log (relay.MMMMM), and then it reaches 
the hot log relay.NNNNN again, in which case, the read 
coordinates are not set to zero, but to the old values.

Additionally, some comments to the source code were added.

sql/rpl_rli.h

@@ -94,6 +94,16 @@ public:
   */
   MYSQL_BIN_LOG relay_log;
   LOG_INFO linfo;
+
+  /*
+   cur_log
+     Pointer that either points at relay_log.get_log_file() or
+     &rli->cache_buf, depending on whether the log is hot or there was
+     the need to open a cold relay_log.
+
+   cache_buf 
+     IO_CACHE used when opening cold relay logs.
+   */
   IO_CACHE cache_buf,*cur_log;
 
   /* The following variables are safe to read any time */