Fix missing NUL terminators in xmlBuf and xmlBuffer functions

* buf.c:
(xmlBufAddLen):
- Change check for remaining space to account for the NUL
  terminator.  When adding a length exactly equal to the number
  of unused bytes, a NUL terminator was not written.
(xmlBufResize):
- Set `buf->use` and NUL terminator when allocating a new
  buffer.
* tree.c:
(xmlBufferResize):
- Set `buf->use` and NUL terminator when allocating a new
  buffer.
(xmlBufferAddHead):
- Set NUL terminator before returning early when shifting
  contents.