refactor mruby method(fix indent. remove temporary value, duplicate procedure)
revert mrb_equal method
Merge pull request #2538 from kkkkkt/refactor-method

refactor mruby method(fix indent. remove temporary value etc)
Fix error handlings in mirb.

mrb_parser_new() and mrb_generate_code() may return NULL.
Merge pull request #2539 from cubicdaiya/issues/mirb_error_handlings

Fix error handlings in mirb.
Fix error hanldlings in read_irep_record().

read_irep_record_1() and read_irep_record() may return NULL.
Fix error handling for mrb_generate_code().

The return value of mrb_generate_code()
must be null-checked before referencing.
Merge pull request #2541 from cubicdaiya/issues/codegen_error_handling

Fix error handling for mrb_generate_code().
Merge pull request #2540 from cubicdaiya/issues/read_irep_record_error_handlings

Fix error hanldlings in read_irep_record().
Use specified macro(NULL) instead of magic-number.
Fix error handling for mrb_pool_realloc().

mrb_pool_alloc() may return NULL.
Fix error handling for mrb_open_allocf().

When DISABLE_GEMS is not defined and
a return value of mrb_open_core() is NULL,
mrb_open_allocf() may cause SEGV.
Fix error handlings for mrb_open_core().
Merge pull request #2544 from cubicdaiya/issues/mrb_open_core_error_handling

Fix error handlings for mrb_open_core().
Merge pull request #2543 from cubicdaiya/issues/mrb_open_allocf_error_handlings

Fix error handling for mrb_open_allocf().
Merge pull request #2542 from cubicdaiya/issues/mrb_pool_realloc_error_handlings

Fix error handling for mrb_pool_realloc().
Merge pull request #2545 from cubicdaiya/issues/use_null_insteadof_zero

Use specified macro(NULL) instead of magic-number.