mesa: Assert variable coming from get_variable() in get_current_attrib

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/main/ff_fragment_shader.cpp

@@ -543,6 +543,7 @@ get_current_attrib(texenv_fragment_program *p, GLuint attrib)
    ir_rvalue *val;
 
    current = p->shader->symbols->get_variable("gl_CurrentAttribFragMESA");
+   assert(current);
    current->data.max_array_access = MAX2(current->data.max_array_access, attrib);
    val = new(p->mem_ctx) ir_dereference_variable(current);
    ir_rvalue *index = new(p->mem_ctx) ir_constant(attrib);