memory corruptions.

The right pointer field of the SEL_ARG structure was not
initialized in the constructor and sometimes that led to
server crashes.

There is no testcase because the bug occurs only when
uninitialized memory has particular values, which can't be
re-created in the test suite.


sql/opt_range.cc:
  Fixed bug #32403.
  
  The eq_tree function requires that SEL_ARG::left and
  SEL_ARG::right are equal to null pointer if SEL_ARG
  type is MAYBE_KEY, but SEL_ARG::right was not initialized
  and contained garbage.