message: tests: Fix invalid buffer size
  This was triggering an buffer read overflow with asan.
diff: @@ -1336,7 +1336,7 @@ TEST(parse_encoding_detect_auto_incomplete)
  { "<\x00\x00n/\x00>\x00", 8, encoding_utf16_le },
  { "\x00<n\x00\x00/\x00>", 8, encoding_utf16_be },
  { "<\x00?n/\x00>\x00", 8, encoding_utf16_le },
  - { "\x00 ", 8, encoding_utf8 },
  + { "\x00 ", 2, encoding_utf8 },
  // <?xml encoding
  { "<?xmC encoding='latin1'?>", 25, encoding_utf8 },
  { "<?xBC encoding='latin1'?>", 25, encoding_utf8 },
  