patch 8.2.1714: text properties corrupted with substitute command

Problem:    Text properties corrupted with substitute command. (Filipe
            Brandenburger)
Solution:   Get the changed line again after using u_savesub(). (closes #6984)
diff --git a/src/textprop.c b/src/textprop.c
index 639c5df..bca0d9e 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -1349,6 +1349,10 @@
 						    && u_savesub(lnum) == FAIL)
 		return FALSE;
 	    dirty = TRUE;
+
+	    // u_savesub() may have updated curbuf->b_ml, fetch it again
+	    if (curbuf->b_ml.ml_line_lnum != lnum)
+		proplen = get_text_props(curbuf, lnum, &props, TRUE);
 	}
 	if (res.can_drop)
 	    continue; // Drop this text property