astdb: crash in sqlite3 during shutdown

When Asterisk is shut down, the astdb_atexit() function releases
(finalize) the previously initiated (prepared) SQL statements in
sqlite3.  Another thread making a subsequent request can cause a
crash in sqlite3.  This patch eliminates that issue by resetting
the statement pointer after it is released/cleared.  The sqlite3
code detects the null pointer, and aborts the operation cleanly.

(closes issue AST-1265)
Reported by: Alexander Hömig
(closes issue ASTERISK-22350)
Reported by: Birger "WIMPy" Harzenetter
Review: https://reviewboard.asterisk.org/r/3078/
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Merged revisions 404344 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 404345 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@404346 65c4cc65-6c06-0410-ace0-fbb531ad65f3