Sync AMTL.

public/amtl/am-string.h

@@ -56,7 +56,7 @@ class AString
     if (other.length_)
       set(other.chars_, other.length_);
     else
-     length_ = 0;
+      length_ = 0;
   }
   AString(Moveable<AString> other)
     : chars_(other->chars_.take()),
@@ -106,17 +106,29 @@ class AString
     return chars()[index];
   }
 
+  void setVoid() {
+    chars_ = NULL;
+    length_ = kInvalidLength;
+  }
+
+  bool isVoid() const {
+    return length_ == kInvalidLength;
+  }
+
   size_t length() const {
+    assert(!isVoid());
     return length_;
   }
 
   const char *chars() const {
     if (!chars_)
-      return "";
+      return isVoid() ? NULL : "";
     return chars_;
   }
 
  private:
+  static const size_t kInvalidLength = (size_t)-1;
+
   void set(const char *str, size_t length) {
     chars_ = new char[length + 1];
     length_ = length;