vrend: Check whether a shader type actually exisits before uploading constants

When changing the from a one-by-one upload to an array upload the test for the
array pointer to the uniform IDs was replaced by testing the uniform array ID
against -1. However, this is not enough, because when the context is allocated
all is cleared to zero, and apparently the structure is re-used in different
invocations of shader programs. Here, one shader might have been removed but
the const field was still allocated resulting in a sigsegv. This happens e.g.
by running

dEQP-GLES31.functional.primitive_bounding_box.wide_points.
  global_state.vertex_tessellation_geometry_fragment.fbo_bbox_larger
  global_state.vertex_tessellation_geometry_fragment.fbo_bbox_smaller
  tessellation_set_per_draw.vertex_tessellation_fragment.default_framebuffer_bbox_equal
  tessellation_set_per_draw.vertex_tessellation_fragment.default_framebuffer_bbox_larger

resulting in
  AddressSanitizer: SEGV on unknown address 0x00000000001c
  The signal is caused by a READ memory access.
    Hint: address points to the zero page.
    #0 0x7f595b9eb875 in vrend_draw_bind_const_shader src/vrend_renderer.c:3677
    #1 0x7f595b9ed37f in vrend_draw_bind_objects src/vrend_renderer.c:3810

Work around this by checking whether the shader type data is allocated.

Fixes: 172c9679b4
    vrend/shader: handle uniforms as array

Tested-by: Jakob Bornecrantz <jakob@collabora.com>
Acked-by: Jakob Bornecrantz <jakob@collabora.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Signed-off-by: Jakob Bornecrantz <jakob@collabora.com>

src/vrend_renderer.c

@@ -3672,6 +3672,7 @@ static void vrend_draw_bind_const_shader(struct vrend_context *ctx,
                                          int shader_type, bool new_program)
 {
    if (ctx->sub->consts[shader_type].consts &&
+       ctx->sub->shaders[shader_type] &&
        (ctx->sub->prog->const_location[shader_type] != -1) &&
        (ctx->sub->const_dirty[shader_type] || new_program)) {
       glUniform4uiv(ctx->sub->prog->const_location[shader_type],