(xlfd_unparse_name): Handle the case of font->size < 0.

(xlfd_unparse_name): Handle the case of font->size < 0.
(mfont__init): Don't initialize Mascent, Mdescent.
(Mascent, Mdescent): Delete these variables.
(mfont_get_prop): Fix previous change.  Check Mfont_ascent and
Mfont_descent instead of Mascent and Mdescent.
(mfont_put_prop): Allow negative size.
(mfont_find): Handle the case of spec->size < 0.  Fix previous
change.
(mfont_list): Handle the case of spec->size < 0.
(mfont_check): Likewise.