Corrected selected check in awful.tag.set_screen()

[Drauthius]

Previously it would check if the awful.tag.selected function existed,
and not whether the tag was actually selected.

[codecov]

Codecov Report

Merging #1600 into master will decrease coverage by -0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1600      +/-   ##
==========================================
- Coverage   80.69%   80.66%   -0.04%     
==========================================
  Files         278      278              
  Lines       17348    17344       -4     
==========================================
- Hits        13999    13990       -9     
- Misses       3349     3354       +5

Flag	Coverage Δ
#functionaltests	64.3% <100%> (-0.05%)	❌
#samples	63.12% <100%> (ø)	✅
#unittests	73.79% <100%> (ø)	✅

Impacted Files	Coverage Δ
lib/awful/tag.lua	71.85% <100%> (-0.2%)	❌
lib/awful/layout/suit/magnifier.lua	63.29% <ø> (-5.39%)	❌
lib/awful/tooltip.lua	85.59% <ø> (-0.43%)	❌

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[blueyed]

Thanks for this fix!

We should add a regression test for this. /cc @Elv13