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.