Bug #41571: MySQL segfaults after innodb recovery

This 5.0 fix will not be pushed into 5.1; a separate fix (from
innodb-5.1-ss4007) will be pushed into 5.1+.

Detailed revision comments:

r4003 | marko | 2009-01-20 16:12:50 +0200 (Tue, 20 Jan 2009) | 10 lines
branches/5.0: rec_set_nth_field(): When the field already is SQL null,
do nothing when it is being changed to SQL null. (Bug #41571)

Normally, MySQL does not pass "do-nothing" updates to the storage engine.
When it does and a column of an InnoDB table that is in ROW_FORMAT=COMPACT
is being updated from NULL to NULL, the InnoDB buffer pool will be corrupted
without this fix.

rb://81 approved by Heikki Tuuri