Fix: a non ascii byte will trigger BadAddress error

Fix: a non ascii byte will trigger BadAddress error

If a byte in DBusString *unescaped isn't a ascii byte, which will be
cast to char (signed char on most of platform), so that's the issue
unsigned char cast to signed char. e.g. "\303\266" is a valid unicode
character, if everything goes right, it will be escaped to "%c3%b6".
However, in fact, it escaped to "%<garbage-byte>3%<garbage-byte>6".

_dbus_string_append_byte_as_hex() take an int parameter, so negative
byte is valid, but cause get a negative index in array. So garbage value
will get. e.g. '\303' --> hexdigits[((signed byte)(-61)) >> 4] is
hexdigits[-4].

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=53499
Sgne-off-by: Chengwei Yang <chengwei.yang@intel.com>
[fixed whitespace -smcv]
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>