handleUnsigned issue

[Daisongcai]

case :
1, add a new int type columu into table .
2.del the master.info log
handleUnsigned will got the panic issue. when it got old binlog rows.
because the table columus hava a new columu but the old log dont update .

before update the table .there have 31 columns .

after update the tbale . there have 32 columns .
so .the slice index will range out of length