Adding assertion for strange bug related with ARGSubtreeRemover. This is not a fix, only a 'early exception'. The bug causes a endless loop in normal BAM, but with BAM+recursion an exception is thrown. The bug occurs with SBE under certain conditions (second state of reachedset is removed through refinement, same reachedset is used several times, special structure of infeasible errorpaths).

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/branches/bamRecursion@14467 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c