patch 8.2.0112: illegal memory access when using 'cindent'

Problem:    Illegal memory access when using 'cindent'.
Solution:   Check for NUL byte. (Dominique Pelle, closes #5470)
diff --git a/src/testdir/test_cindent.vim b/src/testdir/test_cindent.vim
index 2cb3f24..2d78be1 100644
--- a/src/testdir/test_cindent.vim
+++ b/src/testdir/test_cindent.vim
@@ -5251,4 +5251,13 @@
   enew! | close
 endfunc
 
+" this was going beyond the end of the line.
+func Test_cindent_case()
+  new
+  call setline(1, "case x: // x")
+  set cindent
+  norm! f:a:
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab