patch 8.2.1919: assert_fails() setting emsg_silent changes normal execution

Problem:    Assert_fails() setting emsg_silent changes normal execution.
Solution:   Use a separate flag in_assert_fails.
diff --git a/src/buffer.c b/src/buffer.c
index 5cbaaac..c804348 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2154,7 +2154,7 @@
 	if (top_file_num < 0)		// wrap around (may cause duplicates)
 	{
 	    emsg(_("W14: Warning: List of file names overflow"));
-	    if (emsg_silent == 0)
+	    if (emsg_silent == 0 && !in_assert_fails)
 	    {
 		out_flush();
 		ui_delay(3001L, TRUE);	// make sure it is noticed