Fix bug in complexity calculation

By definition exprts have at least size (complexity) 1, so
check for complexity == 1 not complexity == 0.

Author: TGWDB

src/goto-symex/complexity_limiter.cpp

@@ -137,7 +137,7 @@ complexity_limitert::check_complexity(goto_symex_statet &state)
     return complexity_violationt::NONE;
 
   std::size_t complexity = state.guard.as_expr().bounded_size(max_complexity);
-  if(complexity == 0)
+  if(complexity == 1)
     return complexity_violationt::NONE;
 
   auto &current_call_stack = state.call_stack();