fix leak in asserted_formulas::propagate_values() for proof generation mode continuation of issue #342
fix leak in asserted_formulas::propagate_values() for proof generation mode
continuation of issue #342

Signed-off-by: Nuno Lopes <cb3a713351b386ac31772a65bfacab0d57451a15@microsoft.com>

kyledewey/z3,mschlaipfer/z3,kyledewey/z3,seahorn/z3,kyledewey/z3,mschlaipfer/z3,kyledewey/z3,wujunzero/z3,mschlaipfer/z3,wujunzero/z3,seahorn/z3,mschlaipfer/z3,seahorn/z3,kyledewey/z3,seahorn/z3,mschlaipfer/z3,wujunzero/z3,seahorn/z3,wujunzero/z3,mschlaipfer/z3,wujunzero/z3,kyledewey/z3,wujunzero/z3,seahorn/z3