glsl: Check order and uniqueness of interlock functions

With this commit all remaining compilation tests in Piglit for
ARB_fragment_shader_interlock will pass.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Plamena Manolova <plamena.manolova@intel.com>

src/compiler/glsl/glsl_parser_extras.cpp

@@ -197,6 +197,8 @@ _mesa_glsl_parse_state::_mesa_glsl_parse_state(struct gl_context *_ctx,
    this->current_function = NULL;
    this->toplevel_ir = NULL;
    this->found_return = false;
+   this->found_begin_interlock = false;
+   this->found_end_interlock = false;
    this->all_invariant = false;
    this->user_structures = NULL;
    this->num_user_structures = 0;