Use size_t instead of uint when calculating join buffer size, because uint can be overflown on 64-bit platforms and join_buffer_size > 4 GB.

The test case for this bug is a part of the test suite for bug #5731.


sql/sql_select.cc:
  Use size_t instead of uint when calculating join buffer size, because uint can be overflown on 64-bit platforms and join_buffer_size > 4G.