[plt-scheme] ports, make-srcloc and recursive reads

make-srcloc expects non-negative exact integers (or #f) for line, col,
pos, and span. However, (string) ports *can* return such things as 0,
causing the reader to fail in a wrong way.

Example:
> (read/recursive (open-input-string ";") #\. #f)
make-srcloc: expected argument of type <exact positive integer or #f>; given 0

-- vyzo