patch 8.2.4334: command line popup menu not positioned correctly

Problem:    Command line popup menu not positioned correctly.
Solution:   Also use vim_strsize() on the existing text. (Naruhiko Nishino,
            closes #9727)
diff --git a/src/cmdexpand.c b/src/cmdexpand.c
index b37c4f9..2463ed5 100644
--- a/src/cmdexpand.c
+++ b/src/cmdexpand.c
@@ -622,7 +622,7 @@
 	    compl_match_array[i].pum_extra = NULL;
 	    compl_match_array[i].pum_kind = NULL;
 	}
-	compl_startcol = ccline->cmdpos + 1;
+	compl_startcol = vim_strsize(ccline->cmdbuff) + 1;
 	columns = vim_strsize(xp->xp_pattern);
 	if (showtail)
 	{