Fix memory leak in FamilyViewControllerAttributes

[zenangst]

• Add weak to .nextAttributes .and .previousAttributes in
  FamilyViewControllerAttributes

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1eccba1). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #197   +/-   ##
=========================================
  Coverage          ?   81.33%           
=========================================
  Files             ?       11           
  Lines             ?      836           
  Branches          ?        0           
=========================================
  Hits              ?      680           
  Misses            ?      156           
  Partials          ?        0           

Flag	Coverage Δ
#iOS	83.00% <ø> (?)
#macOS	80.14% <ø> (?)
#tvOS	43.00% <ø> (?)

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Impacted Files	Coverage Δ
...ources/Shared/FamilyViewControllerAttributes.swift	84.21% <ø> (ø)

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