Fixed get_height to ensure that SK_MAXHEIGHT-1 is the maximum level we

Fixed get_height to ensure that SK_MAXHEIGHT-1 is the maximum level we
will allocate for a skip list entry as many routines use arrays of
size SK_MAXHEIGHT to hold a set of level pointers which means that a
level of SK_MAXHEIGHT is not valid due to C arrays being zero based.

This led to a number of subtle and hard to locate problems caused by
stack based arrays being overflowed by one entry when a node was
allocated with the maximum level. As each node only has a one in two
million or so chance of getting a level of SK_MAXHEIGHT this didn't
actually happen all that often.

MERGED FROM HEAD

git-svn-id: svn://svn.valgrind.org/valgrind/branches/VALGRIND_2_2_0_BRANCH@2863