This actually fixes a bug. A context's current_rt field may get set to NULL if
that surface gets destroyed or unloaded. In general, the next time that
context is acquired, context_setup_target() will then set everything up again.
However, if a context_acquire() call with a NULL target ends up returning such
a context, context_setup_target() never sets the current_rt field because the
target FindContext() chooses never gets propagated to context_acquire(). This
patch ensures context_setup_target() is never called with a NULL target.