UPDATED TWICE

For multi update it is not allowed to update a column
of a table if that table is accessed through multiple aliases
and either
1) the updated column is used as partitioning key
2) the updated column is part of the primary key 
   and the primary key is clustered

This check is done in unsafe_key_update().

The bug was that for case 2), it was checked whether
updated_column_number == table_share->primary_key 
However, the primary_key variable is the index number of the 
primary key, not a column number.

Prior to this bugfix, the first column was wrongly believed to be
the primary key. The columns covered by an index is found in
table->key_info[idx_number]->key_part. The bugfix is to check if
any of the columns in the keyparts of the primary key are
updated.

The user-visible effect is that for storage engines with
clustered primary key (e.g. InnoDB but not MyISAM) queries
like 
"UPDATE t1 AS A JOIN t2 AS B SET A.primkey=..."
will now error with 
"ERROR HY000: Primary key/partition key update is not allowed 
since the table is updated both as 'A' and 'B'." 
instead of 
"ERROR 1032 (HY000): Can't find record in 't1_tb'"
even if primkey is not the first column in the table. This 
was the intended behavior of bugfix 11764529.