windows: Fix build with --disable-log

On Windows we use UNREFERENCED_PARAMETER for UNUSED (commit 521105f).
UNREFERENCED_PARAMETER evaluates to {(ctx) = (ctx);} which errors out:
  error: lvalue required as left operand of assignment.
in the cases where ctx is NULL.

Closes #1152

[Tormod: Add ctx reference to avoid unused variable warnings]
Signed-off-by: Tormod Volden <debian.tormod@gmail.com>