patch 8.2.5058: input() does not handle composing characters properly

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

src/getchar.c

@@ -669,7 +669,7 @@ stuffReadbuffSpec(char_u *s)
 	}
 	else
 	{
-	    c = mb_ptr2char_adv(&s);
+	    c = mb_cptr2char_adv(&s);
 	    if (c == CAR || c == NL || c == ESC)
 		c = ' ';
 	    stuffcharReadbuff(c);