fixes errors/assertions from buildbot, triggered by previously committed optimization

the AssumptionUseDefinitionCollector is still incomplete (e.g., does not take structs into account), thus, the optimization might produce too weak, even empty interpolants when fields of structs are relevant to errors

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13143 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c