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.
Remove empty lines.
Fix allocation-error-handlings for scope_new().
Add API `mrb_data_init` to initialize `MRB_TT_DATA` tagged instance.
Merge pull request #2546 from cubicdaiya/issues/emptylines

Remove empty lines.
should use non NULL scope for raise_error(); ref #2547
Merge branch 'cubicdaiya-issues/scope_new_error_handlings'
Merge branch 'master' of github.com:mruby/mruby
Merge pull request #2548 from take-cheeze/mrb_data_init

Add API `mrb_data_init` to initialize `MRB_TT_DATA` tagged instance.
Remove a discarded comparison.

mrb_method_search() does not return NULL.
Instead it raises an exception.
So it is not necessary to evaluate
a return value of mrb_method_search().
Unify include guard styles and header comments.
changed to call check_cv_name_str in check_cv_name_sym and adjust indent
use mrb_str_cat_lit() instead of mrb_str_cat_cstr().