glsl_to_tgsi: Make sure the 'shader' member is always initialized

glsl_to_tgsi: Make sure the 'shader' member is always initialized

Fixes the valgrind report below and random crashes with piglit on radeonsi.

==30005== Conditional jump or move depends on uninitialised value(s)
==30005==    at 0xB13584E: st_translate_program (st_glsl_to_tgsi.cpp:5100)
==30005==    by 0xB14698B: st_translate_fragment_program (st_program.c:747)
==30005==    by 0xB14777D: st_get_fp_variant (st_program.c:824)
==30005==    by 0xB11219C: get_color_fp_variant (st_cb_drawpixels.c:1042)
==30005==    by 0xB1131AE: st_DrawPixels (st_cb_drawpixels.c:1154)
==30005==    by 0xAFF8806: _mesa_DrawPixels (drawpix.c:162)
==30005==    by 0x4EB86DB: stub_glDrawPixels (generated_dispatch.c:6640)
==30005==    by 0x4F1DF08: piglit_visualize_image (piglit-util-gl.c:1574)
==30005==    by 0x40691D: draw_image_to_window_system_fb(int, bool) (draw-buffers-common.cpp:733)
==30005==    by 0x406C8B: draw_reference_image(bool, bool) (draw-buffers-common.cpp:854)
==30005==    by 0x40722A: piglit_display (alpha-to-coverage-dual-src-blend.cpp:117)
==30005==    by 0x4EA7168: run_test (piglit_fbo_framework.c:52)

Cc: "10.1 10.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>