The technical problem was that THD::user_var_events_alloc was reset to NULL
from a valid value when a stored program is executed during the PREPARE statement.

The user visible problem was that the server crashed if user issued a PREPARE
statement using some combination of stored functions and user variables.

The fix is to restore THD::user_var_events_alloc to the original value.
This is a minimal fix for 5.5.

More proper patch has been already implemented for 5.6+. It avoids
evaluation of stored functions for the PREPARE phase.

From the user point of view, this bug is a regression, introduced by the patch for WL2649
(Number-to-string conversions), revid: bar@mysql.com-20100211041725-ijbox021olab82nv
However, the code resetting THD::user_var_events_alloc exists even in 5.1.
The WL just changed the way arguments are converted to strings and the bug became visible.