MDEV-18452 ASAN unknown-crash in Field::set_default upon SET bit_column = DEFAULT

Field_bit for BIT(20) uses 2 full bytes in the record,
with additional 4 uneven bits in the "null bit area".

Field::set_default() called from Field_bit::set_default() erroneously
copied 3 bytes instead of 2 bytes from the record with default values.

Changing Field::set_default() to copy pack_length_in_rec() bytes
instead of pack_length() bytes.

mysql-test/r/type_bit.result

@@ -830,3 +830,10 @@ def					COALESCE(val, 1)	246	2	1	Y	32896	0	63
 COALESCE(val, 1)
 0
 DROP TABLE t1;
+#
+# MDEV-18452 ASAN unknown-crash in Field::set_default upon SET bit_column = DEFAULT
+#
+CREATE TABLE t1 (b BIT(20)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (0);
+UPDATE t1 SET b = DEFAULT;
+DROP TABLE t1;

mysql-test/t/type_bit.test

@@ -458,3 +458,13 @@ DROP TABLE t2;
 SELECT COALESCE(val, 1) FROM t1;
 --disable_metadata
 DROP TABLE t1;
+
+
+--echo #
+--echo # MDEV-18452 ASAN unknown-crash in Field::set_default upon SET bit_column = DEFAULT
+--echo #
+
+CREATE TABLE t1 (b BIT(20)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (0);
+UPDATE t1 SET b = DEFAULT;
+DROP TABLE t1;

sql/field.h

@@ -854,7 +854,7 @@ class Field: public Value_source
   {
     my_ptrdiff_t l_offset= (my_ptrdiff_t) (table->s->default_values -
 					  table->record[0]);
-    memcpy(ptr, ptr + l_offset, pack_length());
+    memcpy(ptr, ptr + l_offset, pack_length_in_rec());
     if (maybe_null_in_table())
       *null_ptr= ((*null_ptr & (uchar) ~null_bit) |
 		  (null_ptr[l_offset] & null_bit));