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

Problem:
========
SHOW BINLOG EVENTS FROM <pos> reports following crash when ASAN is enabled.

SEGV on unknown address
in inline_mysql_mutex_destroy
in my_bitmap_free
in Update_rows_log_event::~Update_rows_log_event()

Fix:
===
**Part3: Initialize m_cols_ai.bitmap to NULL**

sql/log_event.cc

@@ -9513,7 +9513,8 @@ Rows_log_event::Rows_log_event(const char *buf, uint event_len,
   uint8 const common_header_len= description_event->common_header_len;
   Log_event_type event_type= (Log_event_type) buf[EVENT_TYPE_OFFSET];
   m_type= event_type;
-  
+  m_cols_ai.bitmap= 0;
+
   uint8 const post_header_len= description_event->post_header_len[event_type-1];
 
   DBUG_PRINT("enter",("event_len: %u  common_header_len: %d  "
@@ -12746,9 +12747,12 @@ void Update_rows_log_event::init(MY_BITMAP const *cols)
 
 Update_rows_log_event::~Update_rows_log_event()
 {
-  if (m_cols_ai.bitmap == m_bitbuf_ai) // no my_malloc happened
-    m_cols_ai.bitmap= 0; // so no my_free in my_bitmap_free
-  my_bitmap_free(&m_cols_ai); // To pair with my_bitmap_init().
+  if (m_cols_ai.bitmap)
+  {
+    if (m_cols_ai.bitmap == m_bitbuf_ai) // no my_malloc happened
+      m_cols_ai.bitmap= 0; // so no my_free in my_bitmap_free
+    my_bitmap_free(&m_cols_ai); // To pair with my_bitmap_init().
+  }
 }