Fix for Mahjong's FieldPointsToGraph

[jjppp]

Inconsistency

The current implementation of Mahjong is inconsistent with its paper. According to [1], if the field f of an object oi may point to another object oj, then there is an edge from oi to oj with label f added to the FieldPointsToGraph. However, currently Mahjong builds edges only for those fields that are loaded in the analyzed program.

MockObjs

ConcurrentModificationException may be thrown when building the FieldPointsToGraph.
In #140 an optimization targeting zero length arrays is introduced by allocating non-functional MockObjs for 0 sized arrays, whose array indices should never point to any objects. Currently FieldPointsToGraph does not check for this situation.

[1] T. Tan, Y. Li, and J. Xue, “Efficient and precise points-to analysis: modeling the heap by merging equivalent automata,” in Proceedings of the 38th ACM SIGPLAN Conference on Programming Language Design and Implementation, in PLDI 2017. New York, NY, USA: Association for Computing Machinery, 2017, pp. 278–291. doi: 10.1145/3062341.3062360.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.

Project coverage is 75.08%. Comparing base (0f4bcf4) to head (ce44bec).

Files with missing lines	Patch %	Lines
...alysis/pta/toolkit/mahjong/FieldPointsToGraph.java	88.23%	0 Missing and 2 partials ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #162      +/-   ##
============================================
+ Coverage     72.80%   75.08%   +2.28%     
- Complexity     4429     4567     +138     
============================================
  Files           480      480              
  Lines         15919    15917       -2     
  Branches       2182     2178       -4     
============================================
+ Hits          11590    11952     +362     
+ Misses         3477     3099     -378     
- Partials        852      866      +14     

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