md_opener_stack: Mark the default branch of switch as unreachable.

We were returning NULL previously, but that would lead to a crash
anyway; all callsites expect to get their respective stack anyway
and anything else would mean we are internally broken.

src/md4c.c

@@ -2555,7 +2555,7 @@ md_opener_stack(MD_CTX* ctx, int mark_index)
         case _T('!'):
         case _T('['):   return &BRACKET_OPENERS;
 
-        default:        return NULL;
+        default:        MD_UNREACHABLE();
     }
 }