Fix race condition with disposal in MemberSingle.tryEmit

[davidmoten]

As reported in #58 there was a race condition where all Observers could be disposed by the time the code hits this line in MemberSingle.tryEmit:

rxjava2-jdbc/rxjava2-pool/src/main/java/org/davidmoten/rx/pool/MemberSingle.java

Line 276 in 841bda1

if (x.index == index && x.observers[index] == o) {

If all observers have been disposed then x.observers[index] throws an ArrayIndexOutOfBoundsException.

The fix is to check the active count of observers.

I've also fixed the message of a Precondtions check, added some comments, and moved another comment to a better place.

[codecov-commenter]

Codecov Report

Merging #59 (71e6af1) into master (21c91fb) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master      #59      +/-   ##
============================================
- Coverage     77.32%   77.28%   -0.04%     
  Complexity      669      669              
============================================
  Files            73       73              
  Lines          2747     2747              
  Branches        240      240              
============================================
- Hits           2124     2123       -1     
  Misses          517      517              
- Partials        106      107       +1     

Impacted Files	Coverage Δ
...main/java/org/davidmoten/rx/pool/MemberSingle.java	90.18% <0.00%> (-0.37%)	⬇️

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