Added a check for allready freed in wrenFreeVm.

If wrenFreeVm was called twice, this would lead to an error, because the memory was allready freed.
The used check is rather naive, suggesting that a valid vm does have at least one methodName set.

src/vm/wren_vm.c

@@ -81,7 +81,7 @@ WrenVM* wrenNewVM(WrenConfiguration* configuration)
 
 void wrenFreeVM(WrenVM* vm)
 {
-  // TODO: Check for already freed.
+  if( vm == NULL || vm->methodNames.count == 0 ) return;
   // Free all of the GC objects.
   Obj* obj = vm->first;
   while (obj != NULL)