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)
diff --git a/src/getchar.c b/src/getchar.c
index 55fad1c..03d90ac 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -669,7 +669,7 @@
 	}
 	else
 	{
-	    c = mb_ptr2char_adv(&s);
+	    c = mb_cptr2char_adv(&s);
 	    if (c == CAR || c == NL || c == ESC)
 		c = ' ';
 	    stuffcharReadbuff(c);