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.

src/register.c

@@ -1245,6 +1245,8 @@ op_yank(oparg_T *oap, int deleting, int mess)
 				// double-count it.
 				bd.startspaces = (ce - cs + 1)
 							  - oap->start.coladd;
+				if (bd.startspaces < 0)
+				    bd.startspaces = 0;
 				startcol++;
 			    }
 			}

src/testdir/test_virtualedit.vim

@@ -88,6 +88,16 @@ func Test_edit_change()
   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()

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1378,
 /**/
     1377,
 /**/