patch 9.1.0579: Ex command is still executed after giving E1247

Problem:  Ex command is still executed after giving E1247.
Solution: Indicate the error properly and set cmd to NULL.
          (zeertzjq)

closes: #15241

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 614ba20..c90d64d 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -4698,6 +4698,7 @@
 		if (n == MAXLNUM)
 		{
 		    emsg(_(e_line_number_out_of_range));
+		    cmd = NULL;
 		    goto error;
 		}
 	    }
@@ -4728,6 +4729,7 @@
 		    if (lnum >= 0 && n >= LONG_MAX - lnum)
 		    {
 			emsg(_(e_line_number_out_of_range));
+			cmd = NULL;
 			goto error;
 		    }
 		    lnum += n;