Add 2 asserts in search-replace implementation.

src/runtime.cc

Index: src/runtime.cc
===================================================================
--- src/runtime.cc	(revision 1985)
+++ src/runtime.cc	(working copy)
@@ -1420,6 +1420,7 @@
 
   void AddElement(Object* element) {
     ASSERT(element->IsSmi() || element->IsString());
+    ASSERT(parts_->length() > part_count_);
     parts_->set(part_count_, element);
     part_count_++;
   }
@@ -1589,6 +1590,7 @@
             if (i > last) {
               parts->Add(ReplacementPart::ReplacementSubString(last, i));
             }
+            ASSERT(capture_ref <= capture_count);
             parts->Add(ReplacementPart::SubjectCapture(capture_ref));
             last = next_index + 1;
           }