Remove redundant null checks.
Extract each expresion in "if" statements. It is for maintainability.
Merge pull request #709 from monaka/pr-remove-redundant-null-checks

Remove redundant null checks.
Merge branch 'master' of github.com:mruby/mruby into pr-strict-error-checking
Fix error handlings.
Add a comment: the usage of MRB_DUMP_GENERAL_FAILURE.
Merge pull request #710 from monaka/pr-strict-error-checking

Strict runtime error checking