1. the crash - I suppose this crash happens in some nss module (what is your glibc package version-release)?
2. pam_unix module doesn't guard against crash in unix_chkpwd - it should probably rather disallow access than allow it in case of the crash. Also when this problem with screensaver happens to you what happens when you try to login on the text console?
I see 2 problems here:
1. the crash - I suppose this crash happens in some nss module (what is your glibc package version-release)?
2. pam_unix module doesn't guard against crash in unix_chkpwd - it should probably rather disallow access than allow it in case of the crash. Also when this problem with screensaver happens to you what happens when you try to login on the text console?