Check for empty substring.

src/heap.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2616 matching lines @@
   WriteBarrierMode mode = cons_string->GetWriteBarrierMode(no_gc);
   cons_string->set_length(length);
   cons_string->set_hash_field(String::kEmptyHashField);
   cons_string->set_first(first, mode);
   cons_string->set_second(second, mode);
   return result;
 }
 
 
 MaybeObject* Heap::AllocateSubString(String* buffer,
-                                int start,
-                                int end,
-                                PretenureFlag pretenure) {
+                                     int start,
+                                     int end,
+                                     PretenureFlag pretenure) {
   int length = end - start;
-
-  if (length == 1) {
+  if (length == 0) {
+    return empty_string();
+  } else if (length == 1) {
     return LookupSingleCharacterStringFromCode(buffer->Get(start));
   } else if (length == 2) {
     // Optimization for 2-byte strings often used as keys in a decompression
     // dictionary.  Check whether we already have the string in the symbol
     // table to prevent creation of many unneccesary strings.
     unsigned c1 = buffer->Get(start);
     unsigned c2 = buffer->Get(start + 1);
     return MakeOrFindTwoCharacterString(this, c1, c2);
   }
 
@@ skipping 3381 matching lines @@
 }
 
 
 void ExternalStringTable::TearDown() {
   new_space_strings_.Free();
   old_space_strings_.Free();
 }
 
 
 } }  // namespace v8::internal