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/ex_cmds.c b/src/ex_cmds.c
index 5b118e5..c38687d 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -5288,6 +5288,8 @@
 
 			    getvcol(curwin, &curwin->w_cursor, &sc, NULL, NULL);
 			    curwin->w_cursor.col = regmatch.endpos[0].col - 1;
+			    if (curwin->w_cursor.col < 0)
+				curwin->w_cursor.col = 0;
 			    getvcol(curwin, &curwin->w_cursor, NULL, NULL, &ec);
 			    if (subflags.do_number || curwin->w_p_nu)
 			    {