[runtime] Make Code::SourcePositionTableIfCollected() read only once

Bug: v8:7790
Change-Id: I8160a3729fcff8037e83eb4566e15b6b0f42781f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2358736Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69428}

src/objects/code-inl.h

@@ -211,9 +211,11 @@ void Code::clear_padding() {
 ByteArray Code::SourcePositionTableIfCollected() const {
   ReadOnlyRoots roots = GetReadOnlyRoots();
   Object maybe_table = source_position_table();
-  if (maybe_table.IsUndefined(roots) || maybe_table.IsException(roots))
+  if (maybe_table.IsUndefined(roots) || maybe_table.IsException(roots)) {
     return roots.empty_byte_array();
-  return SourcePositionTable();
+  }
+  DCHECK(maybe_table.IsByteArray());
+  return ByteArray::cast(maybe_table);
 }
 
 ByteArray Code::SourcePositionTable() const {