add 0 length guard in IsNonArrayIndexInteger

TBR=verwaest@chromium.org

BUG=466086
LOG=N

Review URL: https://codereview.chromium.org/998213002

Cr-Commit-Position: refs/heads/master@{#27151}

src/conversions.cc

@@ -509,7 +509,7 @@ bool IsNonArrayIndexInteger(String* string) {
   uint16_t buffer[kBufferSize];
   int offset = 0;
   const int length = string->length();
-  DCHECK_NE(0, length);
+  if (length == 0) return false;
   // First iteration, check for minus, 0 followed by anything else, etc.
   int to = std::min(offset + kUint32MaxChars, length);
   {

test/cctest/test-conversions.cc

@@ -374,6 +374,7 @@ static void CheckNonArrayIndex(bool expected, const char* chars) {
 TEST(NonArrayIndexParsing) {
   auto isolate = CcTest::i_isolate();
   HandleScope scope(isolate);
+  CheckNonArrayIndex(false, "");
   CheckNonArrayIndex(false, "-");
   CheckNonArrayIndex(false, "0");
   CheckNonArrayIndex(false, "01");