Fix off by 1 problem.

Fix off by 1 problem.

The problem was manifestated only with BIT columns and only when more than
one row was fetched. The problem was coming from the fact that in pre-7.0
times mysqlnd was using a no-copy optimization. This optimization kept the
strings (and also the BIT mask equivalents as strings) in the packet and the
zval referred to them. 7.0+ zvals cannot use no-copy and always copy. Because
of this the allocated memory for the packet was reduced by 1 by the person who
ported the driver, but the starting address of the bit area wasn't reduced.
Because of this the bit_area started at wrong address and the length decoded
wrong.