patch 9.1.1383: completion: 'isexpand' option does not handle space char correct

Problem:  When a space character is used as a trigger in 'isexpand' option
          it doesn't get recognized because skip_to_option_part() skips
          spaces after a comma, treating them as option separators
          rather than option value (after v9.1.1341)
Solution: manually set the part to a space character (glepnir).

closes: #17305

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/insexpand.c b/src/insexpand.c
index bb280fa..5f5a5b9 100644
--- a/src/insexpand.c
+++ b/src/insexpand.c
@@ -3665,7 +3665,15 @@
 
 	while (*p != NUL)
 	{
-	    int len = copy_option_part(&p, part, MAXPATHL, ",");
+	    int	    len = 0;
+	    if (*p == ',' && *(p+1) == ' ' && (*(p+2) == ',' || *(p+2) == NUL))
+	    {
+		part[0] = ' ';
+		len = 1;
+		p++;
+	    }
+	    else
+		len = copy_option_part(&p, part, MAXPATHL, ",");
 
 	    if (len > 0 && len <= col)
 	    {