There was a memory leak under these conditions:
- YYABORT was called in the end-of-rule action of a rule containing expr_lex
- This expr_lex was not bound to any sp_lex_keeper
Bison did not call %destructor <expr_lex> in this case, because its stack
already contained a reduced upper-level rule.
Fixing rules starting with RETURN, CONTINUE, EXIT keywords:
Turning end-of-rule actions with YYABORT into mid-rule actions
by adding an empty trailing { } block. This prevents the upper level
rule from being reduced without calling %destructor <expr_lex>.
In other rules expr_lex is used not immediately before the last
end-of-rule { } block, so they don't need changes.
