patch: fix sql plugin crash

[Maxim Gorbachyov]

Hello.
There is a bug in the sql plugin of the cyrus-sasl library.
First, description. Here is relevant piece from my config:

pwcheck_method: auxprop
auxprop_plugin: sql
sql_engine: sqlite
sql_database: my.db
sql_select: SELECT pwd FROM users WHERE login = '%u'
sql_verbose: yes

I get SEGFAULTs when user with unknown login attempts to authorise.
I've found that sql_auxprop_lookup() uses 'value' and (most important)
'value_len' without previous initialization. Sqlite backend does not
change value_len if nothing was read from db. So we get
utils->prop_set() accessing random memory.

Proposed patch (see attachment) initializes 'value' and 'value_len'
with zeroes. It works for me.
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