locale-util: make SpecialGlyph more like our usual enums

Let's define both an enum and a typedef named SpecialGlyph, the way we
usually do it.

Also, introduce an "invalid" special glyph, assigned to -EINVAL, also
like we always do it. (And handle it somewhat sanely in special_glyph()

src/basic/locale-util.c

@@ -427,8 +427,10 @@ const char *special_glyph(SpecialGlyph code) {
                 },
         };
 
-        assert(code < _SPECIAL_GLYPH_MAX);
+        if (code < 0)
+                return NULL;
 
+        assert(code < _SPECIAL_GLYPH_MAX);
         return draw_table[code >= _SPECIAL_GLYPH_FIRST_EMOJI ? emoji_enabled() : is_locale_utf8()][code];
 }
 

src/basic/locale-util.h

@@ -39,7 +39,7 @@ void init_gettext(void);
 
 bool is_locale_utf8(void);
 
-typedef enum {
+typedef enum SpecialGlyph {
         SPECIAL_GLYPH_TREE_VERTICAL,
         SPECIAL_GLYPH_TREE_BRANCH,
         SPECIAL_GLYPH_TREE_RIGHT,
@@ -70,6 +70,7 @@ typedef enum {
         SPECIAL_GLYPH_LOCK_AND_KEY,
         SPECIAL_GLYPH_TOUCH,
         _SPECIAL_GLYPH_MAX,
+        _SPECIAL_GLYPH_INVALID = -EINVAL,
 } SpecialGlyph;
 
 const char *special_glyph(SpecialGlyph code) _const_;