mesa: Don't try to clear a NULL renderbuffer
mesa: Don't try to clear a NULL renderbuffer

In an ES2 context (or if GL_ARB_ES2_compatibility) is supported, the
framebuffer can be complete with some attachments be missing.  In this
case the _ColorDrawBuffers pointer will be NULL.

Fixes the crash in piglit test fbo-missing-attachment-clear.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=37739
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

NOTE: This is a candidate for the stable branches.

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