mesa: Throw error on glGetActiveUniform inside Begin/End.

Fixes piglit GL_ARB_shader_objeccts/getactiveuniform-beginend.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/main/uniform_query.cpp

@@ -46,6 +46,8 @@ _mesa_GetActiveUniformARB(GLhandleARB program, GLuint index,
    struct gl_shader_program *shProg =
       _mesa_lookup_shader_program_err(ctx, program, "glGetActiveUniform");
 
+   ASSERT_OUTSIDE_BEGIN_END(ctx);
+
    if (!shProg)
       return;