Single-path symex must not fail an invariant when verification succeeds

regression/cbmc/Address_of1/test.desc

@@ -1,8 +1,12 @@
 CORE
 main.c
-
+--stop-on-fail
 ^EXIT=0$
 ^SIGNAL=0$
 ^VERIFICATION SUCCESSFUL$
 --
 ^warning: ignoring
+--
+Adding "--stop-on-fail" should not make a difference for successful
+verification, but previously caused an invariant failure when running CBMC with
+"--paths."

src/goto-checker/single_path_symex_checker.cpp

@@ -75,6 +75,10 @@ operator()(propertiest &properties)
         return result;
     }
 
+    // leave the last worklist item in place for the benefit of output_proof
+    if(worklist->size() == 1)
+      break;
+
     worklist->pop();
   }
 
@@ -83,7 +87,7 @@ operator()(propertiest &properties)
 
   final_update_properties(properties, result.updated_properties);
 
-  // Worklist is empty: we are done.
+  // Worklist just has the last element left: we are done.
   return result;
 }
 
@@ -182,6 +186,8 @@ void single_path_symex_checkert::output_error_witness(
 void single_path_symex_checkert::output_proof()
 {
   // This is incorrect, but the best we can do at the moment.
+  // operator()(propertiest &properties) leaves in place the last worklist item
+  // just for this purpose.
   const path_storaget::patht &resume = worklist->peek();
   output_graphml(resume.equation, ns, options);
 }