draw: assert that we have non-null fragment shader

Instead of just segfaulting.  Recently ran into this.

src/gallium/auxiliary/draw/draw_pipe_pstipple.c

@@ -361,6 +361,8 @@ generate_pstip_fs(struct pstip_stage *pstip)
    tgsi_dump(pstip_fs.tokens, 0);
 #endif
 
+   assert(pstip->fs);
+
    pstip->fs->sampler_unit = transform.freeSampler;
    assert(pstip->fs->sampler_unit < PIPE_MAX_SAMPLERS);
 

src/gallium/auxiliary/draw/draw_pipe_wide_point.c

@@ -229,6 +229,8 @@ widepoint_first_point(struct draw_stage *stage,
       const struct draw_fragment_shader *fs = draw->fs.fragment_shader;
       uint i;
 
+      assert(fs);
+
       wide->num_texcoord_gen = 0;
 
       /* Loop over fragment shader inputs looking for generic inputs