vim-patch:8.2.5058: input() does not handle composing characters properly (#18872)

Problem:    input() does not handle composing characters properly.
Solution:   Use mb_cptr2char_adv() instead of mb_ptr2char_adv().
            (closes vim/vim#10527)
e3a529bc87

Cherry-pick all of Test_input_func() from patch 8.2.0316.

src/nvim/getchar.c

@@ -629,7 +629,7 @@ void stuffReadbuffSpec(const char *s)
       stuffReadbuffLen(s, 3);
       s += 3;
     } else {
-      int c = mb_ptr2char_adv((const char_u **)&s);
+      int c = mb_cptr2char_adv((const char_u **)&s);
       if (c == CAR || c == NL || c == ESC) {
         c = ' ';
       }