Reserve byte for NUL terminator and report errors consistently in xmlBuf and xmlBuffer

This is a follow-up to commit 6c283d83.

* buf.c:
(xmlBufGrowInternal):
- Call xmlBufMemoryError() when the buffer size would overflow.
- Account for NUL terminator byte when using XML_MAX_TEXT_LENGTH.
- Do not include NUL terminator byte when returning length.
(xmlBufAdd):
- Call xmlBufMemoryError() when the buffer size would overflow.

* tree.c:
(xmlBufferGrow):
- Call xmlTreeErrMemory() when the buffer size would overflow.
- Do not include NUL terminator byte when returning length.
(xmlBufferResize):
- Update error message in xmlTreeErrMemory() to be consistent
  with other similar messages.
(xmlBufferAdd):
- Call xmlTreeErrMemory() when the buffer size would overflow.
(xmlBufferAddHead):
- Add overflow checks similar to those in xmlBufferAdd().
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