r300g: fix crash when adding a new relocation
r300g: fix crash when adding a new relocation

No idea why it's so hard to reproduce.
Broken with: c35572352e3e92683988ee8d151b47f4190d62f9

Thanks to Toni Spets for assistance.

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