fix glyph_to_cmap() odd macro results

Some discrepencies between glyph_is_cmap and glyph_to_cmap
arose after b14b830b because the change resulted in
glyph_is_cmap matching on zap beams which weren't accounted
for in the glyph_to_cmap macro. It is unlikely that
glyph_to_cmap will ever be used on such a glyph, but at least
have glyph_to_cmap return a sane value rather than drop through
to the last-resort value (currently NO_GLYPH) which is far
outside the range of the defsyms[] array indices.

src/cmd.c

@@ -4026,6 +4026,13 @@ wiz_display_macros(void)
                         glyph, test);
                  putstr(win, 0, buf);
             }
+            if (glyph_is_cmap_zap(glyph)
+                && !(test >= S_vbeam && test <= S_rslant)) {
+                Sprintf(buf,
+                        "glyph_is_zap(glyph=%d) returned non-zap cmap %d",
+                        glyph, test);
+                 putstr(win, 0, buf);
+            }
             /* check against defsyms array subscripts */
             if (test < 0 || test >= SIZE(defsyms)) {
                 if (!trouble++)