If we can avoid it, do not parse the condition twice
in ternary operators where the then-branch is missing (e.g., (x ? : y)).
Parsing something twice will always fail
if it contains a label.
This fixes #177.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13828 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c