MDEV-10570: Fix -Wmaybe-uninitialized

Rows_log_event::change_to_flashback_event(): Reduce the scope
of the variable swap_buff2, and do not duplicate conditions.

GCC 9.3.0 flagged the -Wmaybe-uninitialized when compiling the
10.5 branch using cmake -DWITH_ASAN=ON -DCMAKE_CXX_FLAGS=-O2

sql/log_event.cc

 /*
    Copyright (c) 2000, 2019, Oracle and/or its affiliates.
-   Copyright (c) 2009, 2019, MariaDB
+   Copyright (c) 2009, 2020, MariaDB
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -3184,7 +3184,7 @@ void Rows_log_event::change_to_flashback_event(PRINT_EVENT_INFO *print_event_inf
   Table_map_log_event *map;
   table_def *td;
   DYNAMIC_ARRAY rows_arr;
-  uchar *swap_buff1, *swap_buff2;
+  uchar *swap_buff1;
   uchar *rows_pos= rows_buff + m_rows_before_size;
 
   if (!(map= print_event_info->m_table_map.get_table(m_table_id)) ||
@@ -3233,7 +3233,7 @@ void Rows_log_event::change_to_flashback_event(PRINT_EVENT_INFO *print_event_inf
       }
       value+= length2;
 
-      swap_buff2= (uchar *) my_malloc(length2, MYF(0));
+      void *swap_buff2= my_malloc(length2, MYF(0));
       if (!swap_buff2)
       {
         fprintf(stderr, "\nError: Out of memory. "
@@ -3241,21 +3241,14 @@ void Rows_log_event::change_to_flashback_event(PRINT_EVENT_INFO *print_event_inf
         exit(1);
       }
       memcpy(swap_buff2, start_pos + length1, length2); // WHERE part
-    }
 
-    if (ev_type == UPDATE_ROWS_EVENT ||
-        ev_type == UPDATE_ROWS_EVENT_V1)
-    {
       /* Swap SET and WHERE part */
       memcpy(start_pos, swap_buff2, length2);
       memcpy(start_pos + length2, swap_buff1, length1);
+      my_free(swap_buff2);
     }
 
-    /* Free tmp buffers */
     my_free(swap_buff1);
-    if (ev_type == UPDATE_ROWS_EVENT ||
-        ev_type == UPDATE_ROWS_EVENT_V1)
-      my_free(swap_buff2);
 
     /* Copying one row into a buff, and pushing into the array */
     LEX_STRING one_row;