fetch1: Fix debug stop again

This fixes a bug which prevents the core from stopping properly.  The
same bug was previously fixed in commit e41cb01bca ("fetch1: Fix
debug stop", 2020-12-19) and reintroduced by commit 0fb207be60
("fetch1: Implement a simple branch target cache", 2020-12-19).

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

fetch1.vhdl

@@ -93,7 +93,7 @@ begin
             end if;
             -- always send the up-to-date stop mark and req
             r.stop_mark <= stop_in;
-            r.req <= not rst;
+            r.req <= not rst and not stop_in;
 	end if;
     end process;
     log_out <= log_nia;