patch 8.1.0794: white space before " -Ntabmove" causes problems

Problem:    White space before " -Ntabmove" causes problems.
Solution:   Skip whitespace. (Ozaki Kiichi, closes #3841)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 62f507c..f60bb34 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -7588,7 +7588,7 @@
 	else
 	{
 	    tab_number = eap->line2;
-	    if (!unaccept_arg0 && **eap->cmdlinep == '-')
+	    if (!unaccept_arg0 && *skipwhite(*eap->cmdlinep) == '-')
 	    {
 		--tab_number;
 		if (tab_number < unaccept_arg0)