Fix seeking to anything except 0 with BufferedStream.

Previously, given a non-zero location to seek to, we would always
seek to the start of the second buffered chunk.

Author: gsnedders

html5lib/inputstream.py

@@ -67,7 +67,7 @@ def seek(self, pos):
         offset = pos
         i = 0
         while len(self.buffer[i]) < offset:
-            offset -= pos
+            offset -= len(self.buffer[i])
             i += 1
         self.position = [i, offset]