glsl: fix segfault linking subroutine uniform with explicit location
glsl: fix segfault linking subroutine uniform with explicit location

Reviewed-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>
Cc: d2f8330c9d523fef404037885ae974fdb513f6b5@lists.freedesktop.org

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler