[runtime] Properly handle thin strings in sliced strings

... when querying internalized strings table and computing hash.

Bug: chromium:974612
Change-Id: I59eed1477c8893cd0ff3eae593da092492668614
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1678418Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Auto-Submit: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62416}

src/objects/objects.cc

@@ -6773,7 +6773,9 @@ Address StringTable::LookupStringIfExists_NoAllocate(Isolate* isolate,
   }
   if (source.IsThinString()) {
     source = ThinString::cast(source).actual();
-    if (start == 0) return source.ptr();
+    if (string.length() == source.length()) {
+      return source.ptr();
+    }
   }
   if (source.IsOneByteRepresentation()) {
     return i::LookupString<uint8_t>(isolate, string, source, start);

src/objects/string.cc

@@ -1335,7 +1335,7 @@ uint32_t String::ComputeAndSetHash() {
   }
   if (string.IsThinString()) {
     string = ThinString::cast(string).actual();
-    if (start == 0) {
+    if (length() == string.length()) {
       set_hash_field(string.hash_field());
       return hash_field() >> kHashShift;
     }