draw: check for out-of-memory conditions in the AA line module.

draw: check for out-of-memory conditions in the AA line module.

To prevent segfaults in the AA line module, the code will check for a
valid pointer to the aaline_stage in the draw context.

Fixes segfault from backtrace:

* aaline_stage_from_pipe
  aaline_delete_fs_state

Reviewed-by: Brian Paul <brianp@vmware.com>