mesa: fix glDeleteBuffers() regression
mesa: fix glDeleteBuffers() regression

This fixes a regression (failed assertion) from commit
c552f273f559968dfd770367e25329baccbcd0c4 which was hit if glDeleteBuffers()
was called on a buffer that was never bound.

NOTE: this is a candidate for the 7.9 branch.

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