make Maria support multiple calls to rnd_init() without an rnd_end()
call in between.


storage/maria/ma_blockrec.c:
  as explained in sql/handler.h, multiple calls to rnd_init() without
  a rnd_end() in between, are possible, and engine must be prepared to
  that. So in _ma_scan_init_block_record(), we allocate a buffer
  only if we have not yet one.