A defect in the subquery substitution code may lead to a server crash:
setting substitution's name should be followed by setting its length
(to keep them in sync).


mysql-test/r/gis.result:
  BUG#12537203 - CRASH WHEN SUBSELECTING GLOBAL VARIABLES IN GEOMETRY FUNCTION ARGUMENTS
    test result.
mysql-test/t/gis.test:
  BUG#12537203 - CRASH WHEN SUBSELECTING GLOBAL VARIABLES IN GEOMETRY FUNCTION ARGUMENTS
    test case.
sql/item_subselect.cc:
  BUG#12537203 - CRASH WHEN SUBSELECTING GLOBAL VARIABLES IN GEOMETRY FUNCTION ARGUMENTS
    set substitution's name length as well as the name itself (to keep them in sync).