C front-end/KnR: generate nondet for missing arguments

The back-end does not know how to handle `nil`, and symex would also
generate nondet in such cases, so do the same in the front-end.

Author: tautschnig

regression/cbmc/KnR1/main.c

@@ -0,0 +1,8 @@
+void a(b)
+{
+  __CPROVER_assert(0, "");
+}
+int main()
+{
+  a();
+}

regression/cbmc/KnR1/test.desc

@@ -0,0 +1,9 @@
+CORE
+main.c
+
+^EXIT=10$
+^SIGNAL=0$
+^VERIFICATION FAILED$
+--
+^warning: ignoring
+Invariant check failed

src/ansi-c/c_typecheck_expr.cpp

@@ -2961,10 +2961,12 @@ void c_typecheck_baset::typecheck_function_call_arguments(
   else if(code_type.is_KnR())
   {
     // We are generous on KnR; any number is ok.
-    // We will in missing ones with "NIL".
-
-    while(parameter_types.size()>arguments.size())
-      arguments.push_back(nil_exprt());
+    // We will fill in missing ones with "nondet".
+    for(std::size_t i = arguments.size(); i < parameter_types.size(); ++i)
+    {
+      arguments.push_back(side_effect_expr_nondett{parameter_types[i].type(),
+                                                   expr.source_location()});
+    }
   }
   else if(code_type.has_ellipsis())
   {