Fix bug in ExtractFixedArray

There is currently no test that shows this bug (it was found as part of the DSL
experiment), but it should get fixed anyway and it will eventually be verified
by the A.p.splice CSA builtin port that is in progress.

Change-Id: I9b6da8dc8f3151ad0b0a06c3ca10d604060efef7
Reviewed-on: https://chromium-review.googlesource.com/890580Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Daniel Clifford <danno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50921}

src/code-stub-assembler.cc

@@ -2990,7 +2990,7 @@ Node* CodeStubAssembler::ExtractFixedArray(Node* fixed_array, Node* first,
   Label if_fixed_double_array(this), empty(this), cow(this),
       done(this, {&var_result, &var_fixed_array_map});
   var_fixed_array_map.Bind(LoadMap(fixed_array));
-  GotoIf(WordEqual(IntPtrOrSmiConstant(0, parameter_mode), count), &empty);
+  GotoIf(WordEqual(IntPtrOrSmiConstant(0, parameter_mode), capacity), &empty);
 
   if (extract_flags & ExtractFixedArrayFlag::kFixedDoubleArrays) {
     if (extract_flags & ExtractFixedArrayFlag::kFixedArrays) {