gallium: fix return value check
gallium: fix return value check

A possible error (-1) was being lost because it was first converted to an
unsigned int and only then checked.

Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>
Signed-off-by: Martina Kollarova <8dd87b772e085009848ac0a920c39792a5f084a0@intel.com>
Reviewed-by: Eric Engestrom <de0b63cf2dfb1fae8ed3c3b4244856a05677d4f0@imgtec.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler