Valgrind warning happpens because of uninitialized null bytes.
In row_sel_push_cache_row_for_mysql() function we fill fetch cache
with necessary field values, row_sel_store_mysql_rec() is called
for this and leaves null bytes untouched.
Later row_sel_pop_cached_row_for_mysql() rewrites table record
buffer with uninited null bytes. We can see the problem from the
test case:
At 'SELECT...' we call row_sel_push...->row_sel_store...->row_sel_pop_cached...
chain which rewrites table->record[0] buffer with uninitialized null bytes.
When we call 'UPDATE...' statement, compare_record uses this buffer and
valgrind warning occurs.
The fix is to init null bytes with default values.


mysql-test/suite/innodb/r/innodb_mysql.result:
  test case
mysql-test/suite/innodb/t/innodb_mysql.test:
  test case
mysql-test/t/ps_3innodb.test:
  enable valgrind testing
storage/innobase/row/row0sel.c:
  init null bytes with default values as they might be
  left uninitialized in some cases and these uninited bytes
  might be copied into mysql record buffer that leads to
  valgrind warnings on next use of the buffer.