Added exception when base index out of range in a normal (non-indexed) segment

* matches what happens in indexed segments

atrcopy/segments.py

@@ -258,6 +258,8 @@ class DefaultSegment(object):
             index = r.get_reverse_index(base_index)
         else:
             index = base_index - r.get_raw_index(0)
+            if not self.is_valid_index(index):
+                raise IndexError("index %d not in this segment" % base_index)
         return index
 
     def tostring(self):