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 <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>

djreep81/glsl-optimizer,benaadams/glsl-optimizer,mcanthony/glsl-optimizer,jbarczak/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,benaadams/glsl-optimizer,jbarczak/glsl-optimizer,wolf96/glsl-optimizer,zz85/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,metora/MesaGLSLCompiler,mapbox/glsl-optimizer,bkaradzic/glsl-optimizer,tokyovigilante/glsl-optimizer,metora/MesaGLSLCompiler,dellis1972/glsl-optimizer,djreep81/glsl-optimizer,tokyovigilante/glsl-optimizer,mapbox/glsl-optimizer,zeux/glsl-optimizer,djreep81/glsl-optimizer,dellis1972/glsl-optimizer,tokyovigilante/glsl-optimizer,zeux/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,wolf96/glsl-optimizer,mapbox/glsl-optimizer,zz85/glsl-optimizer,wolf96/glsl-optimizer,mcanthony/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,zz85/glsl-optimizer,tokyovigilante/glsl-optimizer,dellis1972/glsl-optimizer,benaadams/glsl-optimizer,wolf96/glsl-optimizer,mapbox/glsl-optimizer,djreep81/glsl-optimizer,bkaradzic/glsl-optimizer,zeux/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,dellis1972/glsl-optimizer,wolf96/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,mapbox/glsl-optimizer,metora/MesaGLSLCompiler