Test face name variables are bound before using them in cc-fonts.el

Test face name variables are bound before using them in cc-fonts.el

This fixes bug #58641.

Also, set c-reference-face-name to font-lock-constant face in preference to
c-label-face-name.

* lisp/progmodes/cc-fonts.el (c-preprocessor-face-name, c-label-face-name)
(c-constant-face-name, c-reference-face-name): Test variables with the same
name as font-lock faces are bound before accessing their values.
(c-reference-face-name): Use font-lock-constant face in preference to
c-label-face-name.