patch 8.0.0879: crash when shifting with huge number

Problem:    Crash when shifting with huge number.
Solution:   Check for overflow. (Dominique Pelle, closes #1945)
diff --git a/src/testdir/test_visual.vim b/src/testdir/test_visual.vim
index 371fab5..97b884f 100644
--- a/src/testdir/test_visual.vim
+++ b/src/testdir/test_visual.vim
@@ -18,6 +18,14 @@
   q!
 endfunc
 
+func Test_block_shift_overflow()
+  " This used to cause a multiplication overflow followed by a crash.
+  new
+  normal ii
+  exe "normal \<C-V>876543210>"
+  q!
+endfunc
+
 func Test_dotregister_paste()
   new
   exe "norm! ihello world\<esc>"