linker: Allow consumer stage or producer stage to be NULL

linker: Allow consumer stage or producer stage to be NULL

When linking a separable program that contains only a fragment shader,
the producer will be NULL.  Similar cases will exist with geometry
shaders and, eventually, tessellation shaders.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>