patch 7.4.810

Problem:    With a sequence of commands using buffers in diff mode E749 is
            given. (itchyny)
Solution:   Skip unloaded buffer. (Hirohito Higashi)

src/diff.c

@@ -804,8 +804,8 @@ ex_diffupdate(eap)
     for (idx_new = idx_orig + 1; idx_new < DB_COUNT; ++idx_new)
     {
 	buf = curtab->tp_diffbuf[idx_new];
-	if (buf == NULL)
-	    continue;
+	if (buf == NULL || buf->b_ml.ml_mfp == NULL)
+	    continue; /* skip buffer that isn't loaded */
 	if (diff_write(buf, tmp_new) == FAIL)
 	    continue;
 	diff_file(tmp_orig, tmp_new, tmp_diff);

src/version.c

@@ -741,6 +741,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    810,
 /**/
     809,
 /**/