user-accounts: Fix password dialog crashes

Currently, the password_entry_timeout() callback can be called even after
the dialog disposal, which leads to segfaults. This is because the
corresponding source ids can be cleared in some cases without removing
the sources. Let's prevent direct calls of password_entry_timeout()
without destroying the source ids in order to fix this issue.

Fixes: #760
11 jobs for wip/oholy/password-dialog-crashes in 3 minutes and 59 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #494223
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  Test
passed #494225
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passed #494224
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manual #494230
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tsan
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  Review
manual #494232
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