github PR #1259 - counts in curses menus

Pull request from mkuoppal:  curses menu handling could go out of
bounds accessing array groupaccels[] if strange input gave a false
positive for STDC's isdigit().  Discovered by debug fuzzer.

Failure was triggering an error by the undefined behavior sanitizer.

[randomkey() ought to return int rather than char.]

Closes #1259

win/curses/cursdial.c

@@ -1555,7 +1555,8 @@ menu_get_selections(WINDOW *win, nhmenu *menu, int how)
             }
             /*FALLTHRU*/
         default:
-            if (isdigit(curletter) && !selectors[curletter]
+            if (curletter > 0 && curletter < 256
+                && isdigit(curletter) && !selectors[curletter]
                 && !groupaccels[curletter]) {
                 count = curses_get_count(curletter);
                 /* after count, we know some non-digit is already pending */