As the result of DOUBLE claculations can be bigger
than DBL_MAX constant we use in code, we shouldn't use this constatn
as a biggest possible value.
Particularly the rtree_pick_key function set 'min_area= DBL_MAX' relying
that any rtree_area_increase result will be less so we return valid
key. Though in rtree_area_increase function we calculate the area
of the rectangle, so the result can be 'inf' if the rectangle is
huge enough, which is bigger than DBL_MAX.

Code of the rtree_pick_key modified so we always return a valid key.


myisam/rt_index.c:
  Bug #30286 spatial index cause corruption and server crash!
  
  always set the best_key with the first key we get, so we always return
  somthing valid.
myisam/rt_mbr.c:
  Bug #30286 spatial index cause corruption and server crash!
  
  function comment extended
mysql-test/r/gis-rtree.result:
  Bug #30286 spatial index cause corruption and server crash!
  test result
mysql-test/t/gis-rtree.test:
  Bug #30286 spatial index cause corruption and server crash!
  test case