Fix NULL pointer deref when creating iterator for non-iterable

[danieldk]

Fixes #494.

[danieldk]

Sorry, missed #544.

[codecov]

Codecov Report

Merging #558 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #558      +/-   ##
==========================================
+ Coverage   87.61%   87.62%   +<.01%     
==========================================
  Files          65       65              
  Lines        3449     3451       +2     
==========================================
+ Hits         3022     3024       +2     
  Misses        427      427

Impacted Files	Coverage Δ
src/types/iterator.rs	89.47% <100%> (+1.23%)	⬆️

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[codecov]

Codecov Report

Merging #558 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #558      +/-   ##
==========================================
+ Coverage   87.61%   87.62%   +<.01%     
==========================================
  Files          65       65              
  Lines        3449     3451       +2     
==========================================
+ Hits         3022     3024       +2     
  Misses        427      427

Impacted Files	Coverage Δ
src/types/iterator.rs	89.47% <100%> (+1.23%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 382c79e...ac28a31. Read the comment docs.

[konstin]

Since this passes ci an #544 doesn't, I'm going to take this until we figure what makes #544 fail.

[sebpuetz]

@konstin could you cherrypick this change on 0.7 or release a new minor version including this fix?