MDEV-34061 unix_timestamp(coalesce(timestamp_column)) returns NULL on '1970-01-01 00:00:00.000001'

Field_timestampf::val_native() checked only the
first four bytes to detect zero dates.
That was not enough. Fixing the code to check all packed_length()
bytes to detect zero dates.

mysql-test/main/type_timestamp.result

@@ -1398,5 +1398,16 @@ DROP TABLE t2, t1;
 SET time_zone=DEFAULT;
 SET sql_mode=DEFAULT;
 #
+# MDEV-34061 unix_timestamp(coalesce(timestamp_column)) returns NULL on '1970-01-01 00:00:00.000001'
+#
+SET time_zone='+00:00';
+CREATE TABLE t1 (a TIMESTAMP(6) NULL);
+INSERT INTO t1 VALUES ('1970-01-01 00:00:00.000001');
+SELECT unix_timestamp(a) AS c1, unix_timestamp(coalesce(a)) AS c2 FROM t1;
+c1	c2
+0.000001	0.000001
+DROP TABLE t1;
+SET time_zone=DEFAULT;
+#
 # End of 10.5 tests
 #

mysql-test/main/type_timestamp.test

@@ -940,6 +940,17 @@ DROP TABLE t2, t1;
 SET time_zone=DEFAULT;
 SET sql_mode=DEFAULT;
 
+--echo #
+--echo # MDEV-34061 unix_timestamp(coalesce(timestamp_column)) returns NULL on '1970-01-01 00:00:00.000001'
+--echo #
+
+SET time_zone='+00:00';
+CREATE TABLE t1 (a TIMESTAMP(6) NULL);
+INSERT INTO t1 VALUES ('1970-01-01 00:00:00.000001');
+SELECT unix_timestamp(a) AS c1, unix_timestamp(coalesce(a)) AS c2 FROM t1;
+DROP TABLE t1;
+SET time_zone=DEFAULT;
+
 --echo #
 --echo # End of 10.5 tests
 --echo #

sql/field.cc

@@ -5698,8 +5698,10 @@ my_time_t Field_timestampf::get_timestamp(const uchar *pos,
 bool Field_timestampf::val_native(Native *to)
 {
   DBUG_ASSERT(marked_for_read());
+  char zero[8]= "\0\0\0\0\0\0\0";
+  DBUG_ASSERT(pack_length () <= sizeof(zero));
   // Check if it's '0000-00-00 00:00:00' rather than a real timestamp
-  if (ptr[0] == 0 && ptr[1] == 0 && ptr[2] == 0 && ptr[3] == 0)
+  if (!memcmp(ptr, zero, pack_length()))
   {
     to->length(0);
     return false;