mesa: Return 0 for XFB_VARYING_MAX_LENGTH if no varyings
mesa: Return 0 for XFB_VARYING_MAX_LENGTH if no varyings

v2: Perform this count the same way as elsewhere in this file, per
    Brian Paul's review.

Fixes part of es3conform's transform_feedback_init_defaults test.
NOTE: This is a candidate for the stable branches.
Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>

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