patch 8.2.0126: textprop test fails

Problem:    Textprop test fails.
Solution:   Fix sign in computation.
diff --git a/src/memline.c b/src/memline.c
index 2c41539..aeb8afe 100644
--- a/src/memline.c
+++ b/src/memline.c
@@ -5774,7 +5774,7 @@
 		if (!(offset >= size
 			+ text_end - (int)((dp->db_index[idx]) & DB_INDEX_MASK)
 #ifdef FEAT_PROP_POPUP
-			- (long)(textprop_total - textprop_size)
+			- (long)(textprop_total + textprop_size)
 #endif
 			+ ffdos))
 		    break;