Fix error-path creation in BMCAlgorithm.
We need to get branching values from the SMT solver
even if the ARG is tree-like,
not only if there were merges
(it was not checked if there is a state with multiple children).

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14168 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c