Fix potential deadlock in the SwitchableSubscriptionSubscriber

[cescoffier]

@jponge That fixes the issue you reported with UniRepeat.

[codecov]

Codecov Report

Merging #386 (c239562) into master (6697375) will increase coverage by 0.33%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #386      +/-   ##
============================================
+ Coverage     89.21%   89.54%   +0.33%     
- Complexity     2710     2727      +17     
============================================
  Files           361      363       +2     
  Lines         10501    10520      +19     
  Branches       1302     1306       +4     
============================================
+ Hits           9368     9420      +52     
+ Misses          588      571      -17     
+ Partials        545      529      -16     

Impacted Files	Coverage Δ	Complexity Δ
...subscription/SwitchableSubscriptionSubscriber.java	97.54% <100.00%> (+2.45%)	46.00 <0.00> (+1.00)
...main/java/io/smallrye/mutiny/groups/UniCreate.java	100.00% <0.00%> (ø)	36.00% <0.00%> (-3.00%)
...ny/operators/uni/builders/StateSuppliedtemUni.java	92.30% <0.00%> (ø)	5.00% <0.00%> (?%)
.../mutiny/operators/uni/builders/SuppliedtemUni.java	100.00% <0.00%> (ø)	2.00% <0.00%> (?%)
...mallrye/mutiny/operators/multi/MultiFlatMapOp.java	86.64% <0.00%> (+0.93%)	3.00% <0.00%> (ø%)
...erators/multi/builders/SerializedMultiEmitter.java	88.00% <0.00%> (+1.33%)	26.00% <0.00%> (+1.00%)
...java/io/smallrye/mutiny/helpers/Subscriptions.java	81.21% <0.00%> (+2.76%)	49.00% <0.00%> (+2.00%)
...o/smallrye/mutiny/operators/DefaultUniEmitter.java	89.28% <0.00%> (+3.57%)	11.00% <0.00%> (+1.00%)
...lrye/mutiny/subscription/SerializedSubscriber.java	91.50% <0.00%> (+3.77%)	30.00% <0.00%> (+3.00%)
... and 4 more

[jponge]

Thanks @cescoffier