nv50: fix crash in nv50_pre_pipebuffer_map (nv50_screen->cur_ctx)
nv50: fix crash in nv50_pre_pipebuffer_map (nv50_screen->cur_ctx)

nv50_pre_pipebuffer_map references screen->cur_ctx which points
to freed memory after the context is destroyed.
This crash is easily triggerable by progs/xdemos/glxcontexts.

jbarczak/glsl-optimizer,dellis1972/glsl-optimizer,dellis1972/glsl-optimizer,benaadams/glsl-optimizer,wolf96/glsl-optimizer,adobe/glsl2agal,KTXSoftware/glsl2agal,zeux/glsl-optimizer,mapbox/glsl-optimizer,benaadams/glsl-optimizer,benaadams/glsl-optimizer,benaadams/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,wolf96/glsl-optimizer,bkaradzic/glsl-optimizer,wolf96/glsl-optimizer,mapbox/glsl-optimizer,KTXSoftware/glsl2agal,adobe/glsl2agal,djreep81/glsl-optimizer,KTXSoftware/glsl2agal,zz85/glsl-optimizer,metora/MesaGLSLCompiler,benaadams/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,metora/MesaGLSLCompiler,bkaradzic/glsl-optimizer,adobe/glsl2agal,djreep81/glsl-optimizer,zeux/glsl-optimizer,mapbox/glsl-optimizer,tokyovigilante/glsl-optimizer,KTXSoftware/glsl2agal,djreep81/glsl-optimizer,dellis1972/glsl-optimizer,jbarczak/glsl-optimizer,mapbox/glsl-optimizer,zz85/glsl-optimizer,metora/MesaGLSLCompiler,mapbox/glsl-optimizer,mcanthony/glsl-optimizer,tokyovigilante/glsl-optimizer,KTXSoftware/glsl2agal,bkaradzic/glsl-optimizer,dellis1972/glsl-optimizer,jbarczak/glsl-optimizer,zz85/glsl-optimizer,mcanthony/glsl-optimizer,mcanthony/glsl-optimizer,dellis1972/glsl-optimizer,wolf96/glsl-optimizer,zz85/glsl-optimizer,tokyovigilante/glsl-optimizer,djreep81/glsl-optimizer,adobe/glsl2agal,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,wolf96/glsl-optimizer,zeux/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,zz85/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,adobe/glsl2agal