agent: Fix possible uninitalized use of CTX in simple_pwquery.

* common/simple-pwquery.c (agent_open): Clear CTX even on early error.

(cherry picked from commit bafcf7095159493a656382997f8b0d0bb11a20e8)

• GnuPG-bug-id: T4223
• Signed-off-by: Werner Koch <wk@gnupg.org>