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/message.c b/src/message.c
index 8c8e954..f8efabc 100644
--- a/src/message.c
+++ b/src/message.c
@@ -659,7 +659,7 @@
 	    return TRUE;
 	}
 
-	if (emsg_assert_fails_used && emsg_assert_fails_msg == NULL)
+	if (in_assert_fails && emsg_assert_fails_msg == NULL)
 	{
 	    emsg_assert_fails_msg = vim_strsave(s);
 	    emsg_assert_fails_lnum = SOURCING_LNUM;