MDEV-18046: Assortment of crashes, assertion failures and ASAN errors in mysql_show_binlog_events

Problem:
========
SHOW BINLOG EVENTS FROM <pos> reports following ASAN error

AddressSanitizer: heap-buffer-overflow on address 0x60400002acb8
Load_log_event::copy_log_event(char const*, unsigned long, int,
    Format_description_log_event const*)

Fix:
===
**Part6: Moved the event_len validation to the begin of copy_log_event function**

sql/log_event.cc

@@ -5891,6 +5891,8 @@ int Load_log_event::copy_log_event(const char *buf, ulong event_len,
 {
   DBUG_ENTER("Load_log_event::copy_log_event");
   uint data_len;
+  if ((int) event_len < body_offset)
+    DBUG_RETURN(1);
   char* buf_end = (char*)buf + event_len;
   /* this is the beginning of the post-header */
   const char* data_head = buf + description_event->common_header_len;
@@ -5900,9 +5902,7 @@ int Load_log_event::copy_log_event(const char *buf, ulong event_len,
   table_name_len = (uint)data_head[L_TBL_LEN_OFFSET];
   db_len = (uint)data_head[L_DB_LEN_OFFSET];
   num_fields = uint4korr(data_head + L_NUM_FIELDS_OFFSET);
-	  
-  if ((int) event_len < body_offset)
-    DBUG_RETURN(1);
+
   /*
     Sql_ex.init() on success returns the pointer to the first byte after
     the sql_ex structure, which is the start of field lengths array.
@@ -5911,7 +5911,7 @@ int Load_log_event::copy_log_event(const char *buf, ulong event_len,
                                         buf_end,
                                         (uchar)buf[EVENT_TYPE_OFFSET] != LOAD_EVENT)))
     DBUG_RETURN(1);
-  
+
   data_len = event_len - body_offset;
   if (num_fields > data_len) // simple sanity check against corruption
     DBUG_RETURN(1);