Fix compiler warnings

[yuyichao]

Fix two compiler warnings.

1. In ccalltest, the volatile variable pointing to a function that returns integer is incorrectly declare as a variable pointing to a function that returns volatile integer.
   The compiler gives a warning and I'm 90% sure I've got it right but I'm not a expert on these const/volatile modifiers and please tell me if I got it wrong.
2. A underflow check in flisp is no-op. It can probably be removed since the second check should cover it but it's probably better to make it explicit in the source code.

@JeffBezanson maybe the second one should be cherry-picked back to femtolisp if it wasn't fixed there already?