[FIX] Do not return NaN values or negative values from the sizing manager

[rafaelks]

@RocketChat/ios

Fixes a crash happening on iOS 11 devices, wasn't able to reproduce the issue but figured a way to prevent this from happening again (and updated tests for it).

[codecov]

Codecov Report

Merging #2470 into develop will increase coverage by 0.08%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #2470      +/-   ##
===========================================
+ Coverage    26.24%   26.32%   +0.08%     
===========================================
  Files          456      456              
  Lines        16936    16909      -27     
===========================================
+ Hits          4445     4452       +7     
+ Misses       12491    12457      -34

Impacted Files	Coverage Δ
...at/Controllers/Auth/LoginTableViewController.swift	0% <ø> (ø)	⬆️
.../Views/Cells/Auth/AuthSeparatorTableViewCell.swift	0% <ø> (ø)	⬆️
...t/Views/Cells/Auth/LoginServiceTableViewCell.swift	0% <ø> (ø)	⬆️
...hat/Views/Cells/Chat/ChatMessageDaySeparator.swift	0% <ø> (ø)	⬆️
...ws/Cells/Auth/ShowMoreSeparatorTableViewCell.swift	0% <ø> (ø)	⬆️
....Chat/Controllers/Chat/MessagesSizingManager.swift	70% <100%> (+16.15%)	⬆️

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[cardoso]

👍