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/testdir/test_functions.vim b/src/testdir/test_functions.vim
index af07a64..f55e846 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -1545,6 +1545,10 @@
   call feedkeys(":let c = input('name? ', \"x\\<BS>y\")\<CR>\<CR>", 'xt')
   call assert_equal('y', c)
 
+  " Test for using text with composing characters as default input
+  call feedkeys(":let c = input('name? ', \"ã̳\")\<CR>\<CR>", 'xt')
+  call assert_equal('ã̳', c)
+
   " Test for using <CR> as default input
   call feedkeys(":let c = input('name? ', \"\\<CR>\")\<CR>x\<CR>", 'xt')
   call assert_equal(' x', c)