x-load-color-file: avoid array bounds error

x-load-color-file: avoid array bounds error

x-load-color-file expects each line of input to be of the form
"R G B name".  But if "name" is missing, it would read name[-1],
and if that value is '\n', zero it.
* xfaces.c (Fx_load_color_file): Handle missing color name.