js-on-sx: bail out of array set/length at 2^32-1 instead of padding

arr[4294967295] = 'x' and arr.length = 4294967295 were padding
the SX list with js-undefined for ~4 billion entries — instant
timeout. Per ES spec, indices >= 2^32-1 aren't array indices
anyway (regular properties, which we can't store on lists).
Added (>= i 4294967295) bail clauses to js-list-set! and the
length setter.
built-ins/Array: 21/45 → 23/45 (5 timeouts → 2).
conformance.sh: 148/148.

lib/js/runtime.sx

@@ -2866,6 +2866,7 @@
             ((i (js-num-to-int key)) (n (len lst)))
             (cond
               ((< i 0) nil)
+              ((>= i 4294967295) nil)
               ((< i n) (set-nth! lst i val))
               ((= i n) (append! lst val))
               (else (do (js-pad-list! lst n i) (append! lst val))))))
@@ -2876,6 +2877,7 @@
             ((target (js-num-to-int (js-to-number val))) (n (len lst)))
             (cond
               ((< target 0) nil)
+              ((>= target 4294967295) nil)
               ((> target n) (js-pad-list! lst n target))
               (else nil))))
         (else nil))))