Ensure g_key_file_load_from_file() strips a \r on a 4 KB boundary

Ensure g_key_file_load_from_file() strips a \r on a 4 KB boundary

When g_key_file_parse_data() encountered \n, it was checking the previous
character in the current input buffer for a \r to erase, rather than the
previous character in the parse buffer.  If g_key_file_load_from_file()
was given a file with a \r\n sequence straddling a 4 KB boundary, the \n
would be the first character in the input buffer, so the \r would not be
properly stripped.

Bug #640695.

Found-by: Jan Harkes <jaharkes@cs.cmu.edu>