contrib/plugins: fix coverity warning in hotblocks

Coverity complains that we have an unbalance use of mutex leading to
potential deadlocks.

Fixes: CID 1519048
Fixes: a208ba0 ("tests/plugin: add a hotblocks plugin")
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231009164104.369749-26-alex.bennee@linaro.org>

contrib/plugins/hotblocks.c

@@ -69,8 +69,8 @@ static void plugin_exit(qemu_plugin_id_t id, void *p)
         }
 
         g_list_free(it);
-        g_mutex_unlock(&lock);
     }
+    g_mutex_unlock(&lock);
 
     qemu_plugin_outs(report->str);
 }