patch 8.0.0374: invalid memory access when using :sc in Ex mode

Problem:    Invalid memory access when using :sc in Ex mode. (Dominique Pelle)
Solution:   Avoid the column being negative.  Also fix a hang in Ex mode.
diff --git a/src/testdir/test_substitute.vim b/src/testdir/test_substitute.vim
index a3bc04d..f2dfdc7 100644
--- a/src/testdir/test_substitute.vim
+++ b/src/testdir/test_substitute.vim
@@ -106,3 +106,11 @@
     endfor
   endfor
 endfunction
+
+func Test_substitute_repeat()
+  " This caused an invalid memory access.
+  split Xfile
+  s/^/x
+  call feedkeys("Qsc\<CR>y", 'tx')
+  bwipe!
+endfunc