Use a special lookup function for DROP, which doesn't attempt to parse the
  definition.


mysql-test/r/sp-destruct.result:
  Updated test result for BUG#16303.
mysql-test/t/sp-destruct.test:
  Added test case for BUG#16303.
sql/sp.cc:
  New function sp_routine_exists_in_table() for DROP PROCEDURE/FUNCTION; which doesn't
  want to parse the definition, only know if it exists.
  
  Renamed sp_exists_routine to sp_exist_routines and added comment,
  and changed the misnamed parameter/variable 'tables'/'table' to
  'routines'/'routine'.
sql/sp.h:
  New function sp_routine_exists_in_table() for DROP PROCEDURE/FUNCTION.
  
  Renamed sp_exists_routine to sp_exist_routines,
  and changed the misnamed parameter 'tables' to 'routines'.
sql/sql_acl.cc:
  Call to sp_exists_routine() renamed to sp_exist_routines().
sql/sql_parse.cc:
  Use the new sp_routine_exists_in_table() instead of sp_find_routine(), since we don't
  want the routine definition parsed when doing DROP PROCEDURE/FUNCTION.