Changed default thread stack to 288K to get better memory missalignment between stacks of different threads (should speed up things) and to get a bit extra safety.
In maria_open(), don't allocate big arrays on stack as this may lead to stack overflow.
This fixes a valgrind warning detected by buildbot

include/my_pthread.h:
  Changed default thread stack to 288K to get better memory missalignment between stacks of different threads (should speed up things) and to get a bit extra safety.
storage/maria/ma_open.c:
  In maria_open(), don't allocate big arrays on stack as this may lead to stack overflow.