patch 9.0.1378: illegal memory access when using virtual editing

Problem:    Illegal memory access when using virtual editing.
Solution:   Make sure "startspaces" is not negative.
diff --git a/src/testdir/test_virtualedit.vim b/src/testdir/test_virtualedit.vim
index 71cea42..edaae67 100644
--- a/src/testdir/test_virtualedit.vim
+++ b/src/testdir/test_virtualedit.vim
@@ -88,6 +88,16 @@
   set virtualedit=
 endfunc
 
+func Test_edit_special_char()
+  new
+  se ve=all
+  norm a0
+  sil! exe "norm o00000\<Nul>k<a0s"
+
+  bwipe!
+  set virtualedit=
+endfunc
+
 " Tests for pasting at the beginning, end and middle of a tab character
 " in virtual edit mode.
 func Test_paste_in_tab()