patch 9.1.1190: C indentation does not detect multibyte labels

Problem:  C indentation does not detect multibyte labels
Solution: Correctly skip over multibyte characters
          (Anttoni Erkkilä)

closes: #16770

Signed-off-by: Anttoni Erkkilä <anttoni.erkkila@protonmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_indent.vim b/src/testdir/test_indent.vim
index 0f0f9da..57f5206 100644
--- a/src/testdir/test_indent.vim
+++ b/src/testdir/test_indent.vim
@@ -109,6 +109,19 @@
   close!
 endfunc
 
+func Test_userlabel_indent()
+  new
+  call setline(1, ['{', 'label:'])
+  normal GV=
+  call assert_equal('label:', getline(2))
+
+  call setline(2, 'läbél:')
+  normal GV=
+  call assert_equal('läbél:', getline(2))
+
+  close!
+endfunc
+
 " Test for 'copyindent'
 func Test_copyindent()
   new