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().