fixed wrong resetting upstream flags in #T_NGX_HTTP_UPSTREAM_RANDOM

[chobits]

• This bug was introduced by Bugfix: a segmentation fault might occur in master process. #1379.
• Found by test case of dyups module: the first test case of dyups.t failed.

[chobits]

case result:

$ TEST_NGINX_UNSAFE=yes TEST_NGINX_BINARY=/home/xiaochen/work/ctengine/objs/nginx prove -I /home/xiaochen/work/ctengine/tests/nginx-tests/nginx-tests/lib tests/nginx-tests/tengine-tests/dyups.t
tests/nginx-tests/tengine-tests/dyups.t .. ok
All tests successful.
Files=1, Tests=78, 10 wallclock secs ( 0.03 usr  0.00 sys +  0.24 cusr  0.21 csys =  0.48 CPU)
Result: PASS

$ TEST_NGINX_UNSAFE=yes TEST_NGINX_BINARY=/home/xiaochen/work/ctengine/objs/nginx prove -I /home/xiaochen/work/ctengine/tests/nginx-tests/nginx-tests/lib ./tests/nginx-tests/nginx-tests/upstream_random.t
./tests/nginx-tests/nginx-tests/upstream_random.t .. ok
All tests successful.
Files=1, Tests=14, 18 wallclock secs ( 0.00 usr  0.01 sys +  0.09 cusr  0.02 csys =  0.12 CPU)
Result: PASS

[wangfakang]

LGTM