patch 8.1.2180: Error E303 is not useful when 'directory' is empty

Problem:    Error E303 is not useful when 'directory' is empty.
Solution:   Skip the error message. (Daniel Hahler, #5067)
diff --git a/src/memline.c b/src/memline.c
index c77d1df..a26e401 100644
--- a/src/memline.c
+++ b/src/memline.c
@@ -804,9 +804,9 @@
 	}
     }
 
-    if (mfp->mf_fname == NULL)		/* Failed! */
+    if (*p_dir != NUL && mfp->mf_fname == NULL)
     {
-	need_wait_return = TRUE;	/* call wait_return later */
+	need_wait_return = TRUE;	// call wait_return later
 	++no_wait_return;
 	(void)semsg(_("E303: Unable to open swap file for \"%s\", recovery impossible"),
 		    buf_spname(buf) != NULL ? buf_spname(buf) : buf->b_fname);