Change to always use locals->car->car.

When p->locals->car is not NULL and p->locals->car->car is NULL,
local_add_f dose not append sym to p->locals->car->car.
This commit change to append sym to p->locals->car->car.
This will close #2333.
Fixed dead assignment in 'backtrace.c' reported by 'clang-analyzer'
Fixed dead assignment in 'parse.y' reported by 'clang-analyzer'
Fixed dead assignment in 'vm.c' reported by 'clang-analyzer'
Fixed possible division by zero in 'codegen.c' reported by 'clang-analyzer'
Fixed dead increment in 'dump.c' reported by 'clang-analyzer'
Fixed dereference to null pointer in 'codegen.c' reported by 'clang-analyzer'
Fixed dereference to null pointer in 'codegen.c' reported by 'clang-analyzer'
Revert "Fixed dereference to null pointer in 'codegen.c' reported by 'clang-analyzer'"

This reverts commit 29f14e728d319d9fc80702ba78959c1957944767.
Fixed dead assignment in 'parser.y' reported by 'clang-analyzer'
Fixed dead assignment in 'parser.y' reported by 'clang-analyzer'
Fixed dead assignment in 'parser.y' reported by 'clang-analyzer'
Merge branch 'master' into fix-clang-analizer-warnings

* master:
  update add_confict document; ref #2391
  Update document by regenerating it.
  Add :doc task to run document generator.
  Add document for `add_conflict` and `add_conflicts`.
Fix conflict doc since there is only `add_conflict` method.
Merge pull request #2394 from take-cheeze/fix_conflict_doc

Fix conflict doc since there is only `add_conflict` method.
Using 'mrb_assert' instead of an returning 0 while checking s->irep in codegen.c.
Rolling back change induced because of 'clang-analizer' report.
Merge pull request #2393 from goyox86/fix-clang-analizer-warnings

Fix clang analizer warnings
Merge pull request #2344 from yui-knk/fix-locals

Change to always use locals->car->car.