patch 9.0.0622: matchaddpos() can get slow when adding many matches

Problem:    matchaddpos() can get slow when adding many matches.
Solution:   Update the next available match ID when manually picking an ID and
            remove check if the available ID can be used. (idea by Rick Howe)
diff --git a/src/window.c b/src/window.c
index d4580e4..f4e9750 100644
--- a/src/window.c
+++ b/src/window.c
@@ -5143,8 +5143,7 @@
 #endif
     unblock_autocmds();
 #ifdef FEAT_SEARCH_EXTRA
-    new_wp->w_match_head = NULL;
-    new_wp->w_next_match_id = 4;
+    new_wp->w_next_match_id = 1000;  // up to 1000 can be picked by the user
 #endif
     return new_wp;
 }