Fixed bug mdev-3866.

The invalid implementation of the method Field_bit::cmp_max could
trigger a valgrind complain or could lead to incorrect statistical
data when collecting engine-independent statistics on BIT fields.

mysql-test/r/stat_tables_partition.result

+#
+# Bug mdev-3866: valgrind complain from ANALYZE on a table with BIT field
+# 
+SET use_stat_tables = 'preferably';
+CREATE TABLE t1 (pk int PRIMARY KEY, a bit(1), INDEX idx(a)
+) ENGINE=MyISAM PARTITION BY KEY(pk) PARTITIONS 2;
+INSERT INTO t1 VALUES (1,1),(2,0),(3,0),(4,1);
+ANALYZE TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+SET use_stat_tables = DEFAULT;
+DROP TABLE t1;

mysql-test/t/stat_tables_partition.test

+--source include/have_partition.inc
+
+--echo #
+--echo # Bug mdev-3866: valgrind complain from ANALYZE on a table with BIT field
+--echo # 
+
+SET use_stat_tables = 'preferably';
+
+CREATE TABLE t1 (pk int PRIMARY KEY, a bit(1), INDEX idx(a)
+) ENGINE=MyISAM PARTITION BY KEY(pk) PARTITIONS 2;
+INSERT INTO t1 VALUES (1,1),(2,0),(3,0),(4,1);
+
+ANALYZE TABLE t1;
+
+SET use_stat_tables = DEFAULT;
+
+DROP TABLE t1;

sql/field.cc

@@ -8371,7 +8371,9 @@ int Field_bit::cmp_max(const uchar *a, const uchar *b, uint max_len)
     if ((flag= (int) (bits_a - bits_b)))
       return flag;
   }
-  return memcmp(a, b, field_length);
+  if (!bytes_in_rec)
+    return 0;
+  return memcmp(a, b, bytes_in_rec);
 }