mesa: fix deleting the dummy ATI_fs

mesa: fix deleting the dummy ATI_fs

The DummyShader is used by GenFragmentShadersATI() as a placeholder to
mark IDs as allocated. Context cleanup wants to delete everything in
ctx->Shared->ATIShaders, and crashes on these placeholders with this
backtrace:
==15060== Invalid free() / delete / delete[] / realloc()
==15060==    at 0x482F478: free (vg_replace_malloc.c:530)
==15060==    by 0x57694F4: _mesa_delete_ati_fragment_shader (atifragshader.c:68)
==15060==    by 0x58B33AB: delete_fragshader_cb (shared.c:208)
==15060==    by 0x5838836: _mesa_HashDeleteAll (hash.c:295)
==15060==    by 0x58B365F: free_shared_state (shared.c:377)
==15060==    by 0x58B3BC2: _mesa_reference_shared_state (shared.c:469)
==15060==    by 0x578687F: _mesa_free_context_data (context.c:1366)
==15060==    by 0x595E9EC: st_destroy_context (st_context.c:642)
==15060==    by 0x5987057: st_context_destroy (st_manager.c:772)
==15060==    by 0x5B018B6: dri_destroy_context (dri_context.c:217)
==15060==    by 0x5B006D3: driDestroyContext (dri_util.c:511)
==15060==    by 0x4A1CBE6: dri3_destroy_context (dri3_glx.c:170)
==15060==  Address 0x7b5dae0 is 0 bytes inside data symbol "DummyShader"

Also, DeleteFragmentShadersATI() should not assert on DummyShader, just
remove the hash entry.

Normally one would define a shader after GenFragmentShadersATI(), and
BindFragmentShaderATI() replaces the placeholder with a real object.
However, the specification doesn't say that one has to define a shader
for each allocated ID.

Signed-off-by: Miklós Máté <mtmkls@gmail.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>