Add missing write barriers to StructLayout#initialize

[KJTsanaktsidis]

When setting fields on layout in struct_layout_initialize which contain Ruby VALUEs, we must do so through the RB_OBJ_WRITE macro because it the class is declared as write-barrier protected.

Fixes #1079

[JasonLunn]

Do the other uses of rb_hash_new and rb_ary_new2 throughout the gem need similar handling? Are there any other object initialization calls that to be on the lookout for?

[KJTsanaktsidis]

Do the other uses of rb_hash_new and rb_ary_new2 throughout the gem need similar handling? Are there any other object initialization calls that to be on the lookout for?

This may well be the case - I was planning to do a more thorough pass over the weekend hopefully

[larskanis]

Thank you for your investigation and the patch! For completeness I added RB_WRITE_OBJ also to the Qnil assignments above in commit d006232.

[JasonLunn]

Thanks, all!

@larskanis - can you release an updated release candidate to RubyGems?

[larskanis]

ffi-1.17.0.rc2 is released.

[JasonLunn]

Things seem like they're significantly improved! I no longer see the same error in #1079.

Will there be an RC3, or will the next release be 1.17.0?