Allow zero-length buffers in jv_parser_set_buf()

Allow zero-length buffers in jv_parser_set_buf()

If the caller is at EOF and has no more bytes to feed the parser, how is
the parser to be told about the EOF condition?  A small fix to allow
zero-length buffers in jv_parser_set_buf() fixes this problem (it also
makes it easier to deal with async I/O: feed the parser whatever is
available, including nothing).