MDEV-32140: Valgrind/MSAN warnings in dynamic_column_update_move_left

Do not manipulate empty dynamic column, just better return empty dynamic column from the begining.
(it is also optimisation)

mysql-test/main/dyncol.result

@@ -1956,3 +1956,15 @@ DROP TABLE t1;
 #
 # End of 10.2 tests
 #
+#
+# MDEV-32140: Valgrind/MSAN warnings in dynamic_column_update_move_left
+#
+SELECT COLUMN_GET(COLUMN_ADD(COLUMN_CREATE(1,10),2,NULL,1,NULL),3 AS INTEGER);
+COLUMN_GET(COLUMN_ADD(COLUMN_CREATE(1,10),2,NULL,1,NULL),3 AS INTEGER)
+NULL
+SELECT HEX(COLUMN_ADD(COLUMN_CREATE(1,10),2,NULL,1,NULL));
+HEX(COLUMN_ADD(COLUMN_CREATE(1,10),2,NULL,1,NULL))
+000000
+#
+# End of 10.4 tests
+#

mysql-test/main/dyncol.test

@@ -986,3 +986,13 @@ DROP TABLE t1;
 --echo #
 --echo # End of 10.2 tests
 --echo #
+
+--echo #
+--echo # MDEV-32140: Valgrind/MSAN warnings in dynamic_column_update_move_left
+--echo #
+SELECT COLUMN_GET(COLUMN_ADD(COLUMN_CREATE(1,10),2,NULL,1,NULL),3 AS INTEGER);
+SELECT HEX(COLUMN_ADD(COLUMN_CREATE(1,10),2,NULL,1,NULL));
+
+--echo #
+--echo # End of 10.4 tests
+--echo #

mysys/ma_dyncol.c

@@ -2872,6 +2872,13 @@ dynamic_column_update_move_left(DYNAMIC_COLUMN *str, PLAN *plan,
 
   write= (uchar *)str->str + FIXED_HEADER_SIZE;
   set_fixed_header(str, (uint)new_offset_size, new_column_count);
+  if (!new_column_count)
+  {
+    // No records left
+    DBUG_ASSERT(new_header_size == 0);
+    str->length= FIXED_HEADER_SIZE;
+    return ER_DYNCOL_OK;
+  }
 
   /*
     Move headers first.