When CURSOR parameters get parsed, their sp_assignment_lex instances
(one instance per parameter) get collected to List<sp_assignment_lex>.

These instances get linked to sphead only in the end of the list.
If a syntax error happened in the middle of the parameter list,
these instances were not deleted, which caused memory leaks.

Fix:

using a Bison %destructor to free rules of the <sp_assignment_lex_list>
type (on syntax errors).

Afte the fix these sp_assignment_lex instances from CURSOR parameters
deleted as follows:

- If the CURSOR statement was fully parsed, then these instances
  get properly linked to sp_head structures, so they are deleted
  during ~sp_head (this did not change)

- If the CURSOR statement failed on a syntax error, then by Bison's
  %destructor (this is being added in the current patch).