RTree keys are really different from BTree and need specific
paramters to be set by optimizer to work.
Sometimes optimizer doesn't set those properly.
Here we decided just to add code to check that the parameters
are correct. Hope to fix optimizer sometimes.


myisam/mi_range.c:
  return the error if min_key is NULL
mysql-test/r/gis-rtree.result:
  test result
mysql-test/t/gis-rtree.test:
  test case