recommit e731d8aafa98bbf9d30f83e65f28b7718be55834.
recommit e731d8aafa98bbf9d30f83e65f28b7718be55834.

This got lost with glsl-compiler-1 merge, it fixes segfaults when using
ATI_fragment_shader, which uses the ProgramStringNotify mechanism but doesn't
have a valid program pointer.

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