Valgrind warning for uninitialised bytes in sum_distinct test

[Patrick Crews]

We are seeing a valgrind warning about uninitialised bytes in test sum_distinct.
This appears to have started with revision# 1663:
https://lists.launchpad.net/drizzle-benchmark/msg03264.html

CURRENT_TEST: main.sum_distinct
==5383== Thread 16:
==5383== Syscall param write(buf) points to uninitialised byte(s)
==5383== at 0x4372C0B: ??? (in /lib/tls/i686/cmov/libpthread-2.10.1.so)
==5383== by 0x82C2EB9: drizzled::internal::my_b_flush_io_cache(drizzled::internal::st_io_cache*, int) (mf_iocache.cc:873)
==5383== by 0x82C32FD: drizzled::internal::_my_b_write(drizzled::internal::st_io_cache*, unsigned char const*, unsigned int) (mf_iocache.cc:751)
==5383== by 0x8153F97: drizzled::write_keys(drizzled::st_sort_param*, unsigned char**, unsigned int, drizzled::internal::st_io_cache*, drizzled::internal::st_io_cache*) (filesort.cc:682)
==5383== by 0x8156C50: drizzled::filesort(drizzled::Session*, drizzled::Table*, drizzled::st_sort_field*, unsigned int, drizzled::optimizer::SqlSelect*, unsigned long long, bool, unsigned long long*) (filesort.cc:589)
==5383== by 0x8261438: drizzled::create_sort_index(drizzled::Session*, drizzled::Join*, drizzled::order_st*, unsigned long long, unsigned long long, bool) (sql_select.cc:5298)
==5383== by 0x81D200C: drizzled::Join::exec() (join.cc:1662)
...