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/normal.c b/src/normal.c
index 89eaabc..af4d3b2 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -1154,6 +1154,7 @@
 	    && stuff_empty()
 	    && typebuf_typed()
 	    && emsg_silent == 0
+	    && !in_assert_fails
 	    && !did_wait_return
 	    && oap->op_type == OP_NOP)
     {