Merge upstream - #55

Fuse step and column fetch into one CGO crossing in Next

sqlite3_set_authorizer takes db->mutex, while query cancellation takes
c.mu before calling sqlite3_interrupt. Holding c.mu across the call
could block cancellation of a running query, so use authorizerMu.

…authorizers

fix: release replaced authorizer callbacks

…lose

SQLite dereferences the connection handle without checking it, so every
*SQLiteConn method that passes c.db into the C API segfaulted once Close
had released the handle and set it to nil. This is reachable from
ordinary code: a connection captured from a ConnectHook outlives the
*sql.DB, and calling any of these methods on it killed the process.

Guard the affected methods with the existing dbConnOpen helper, which
reads the handle under mu and is not held across the C call, so it does
not reintroduce the lock-order inversion that RegisterAuthorizer avoids
with its own mutex. Methods that return an error report errConnClosed,
the callback registrations that have no error to report become no-ops,
and the accessors report a documented zero value.

Affected: RegisterCollation, RegisterCommitHook, RegisterRollbackHook,
RegisterUpdateHook, RegisterAuthorizer, RegisterFunc,
RegisterAggregator, AutoCommit, GetFilename, GetLimit, SetLimit,
SetFileControlInt and SetFileControlInt64.

Return an error instead of crashing when a connection is used after Close

Since 4954048, Exec without arguments takes a fast path that steps each
statement without checking its parameters, so a statement whose
arguments are missing ran with them bound as NULL instead of failing
with "not enough args to execute query". Leave such a statement
unexecuted in that path and return the same error as the regular path.

Fixes mattn#1470

…ng-params

Report missing arguments when Exec has no arguments

Upgrade SQLite to version 3540100