Streamline and fix vector-size checks

Streamline and fix vector-size checks

* src/alloc.c (VECTOR_ELTS_MAX): New constant.
(allocate_vectorlike): LEN now must be positive.  Assume LEN
is in range.  All callers changed.
(allocate_vector): Arg is now ptrdiff_t, not EMACS_INT.
All callers changed.  Return zero vector here, not in
allocate_vectorlike.
* src/lisp.h (make_uninit_vector): Simplify.
* src/xwidget.c (webkit_js_to_lisp):
Check for overflow in ptrdiff_t calculations.