[bigint] Fix cornercase in BigInt.asUintN

One of the early (no-op) returns forgot to check whether the
number of existing digits equals the number of required digits.

Bug: chromium:936506
Change-Id: Ic9a5b927306de3cd6b26662785ac11d866e12026
Reviewed-on: https://chromium-review.googlesource.com/c/1493133Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59929}

src/objects/bigint.cc

@@ -2261,9 +2261,8 @@ MaybeHandle<BigInt> BigInt::AsUintN(Isolate* isolate, uint64_t n,
   int needed_length = static_cast<int>((n + kDigitBits - 1) / kDigitBits);
   if (x->length() < needed_length) return x;
   int bits_in_top_digit = n % kDigitBits;
-  if (bits_in_top_digit == 0) {
-    if (x->length() == needed_length) return x;
-  } else {
+  if (x->length() == needed_length) {
+    if (bits_in_top_digit == 0) return x;
     digit_t top_digit = x->digit(needed_length - 1);
     if ((top_digit >> bits_in_top_digit) == 0) return x;
   }

test/mjsunit/harmony/bigint/as-int-n.js

@@ -297,4 +297,8 @@
 }{
   assertThrows(() => BigInt.asUintN(3, 12), TypeError);
   assertEquals(4n, BigInt.asUintN(3, "12"));
+}{
+  // crbug.com/936506
+  assertEquals(1n, BigInt.asUintN(15, 0x100000001n));
+  assertEquals(1n, BigInt.asUintN(15, 0x10000000000000001n));
 }