mesa: minor fixes in _mesa_free_shader_program_data() and _mesa_link_program()

From master:
Set shProg->NumShaders = 0 after freeing program's list of shaders.
Set _NEW_PROGRAM in _mesa_link_program() to fix a state validation bug.

src/mesa/shader/shader_api.c

@@ -126,6 +126,8 @@ _mesa_free_shader_program_data(GLcontext *ctx,
    for (i = 0; i < shProg->NumShaders; i++) {
       _mesa_reference_shader(ctx, &shProg->Shaders[i], NULL);
    }
+   shProg->NumShaders = 0;
+
    if (shProg->Shaders) {
       _mesa_free(shProg->Shaders);
       shProg->Shaders = NULL;
@@ -1008,6 +1010,8 @@ _mesa_link_program(GLcontext *ctx, GLuint program)
       return;
    }
 
+   FLUSH_VERTICES(ctx, _NEW_PROGRAM);
+
    _slang_link(ctx, program, shProg);
 }