i965/cfg: Assert that cur_do/while/if pointers are non-NULL.

Coverity sees that the functions immediately below the new assertions
dereference these pointers, but is unaware that an ENDIF always follows
an IF, etc.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/mesa/drivers/dri/i965/brw_cfg.cpp

@@ -231,6 +231,7 @@ cfg_t::cfg_t(exec_list *instructions)
          if (cur_else) {
             cur_else->add_successor(mem_ctx, cur_endif);
          } else {
+            assert(cur_if != NULL);
             cur_if->add_successor(mem_ctx, cur_endif);
          }
 
@@ -299,6 +300,7 @@ cfg_t::cfg_t(exec_list *instructions)
          inst->exec_node::remove();
          cur->instructions.push_tail(inst);
 
+         assert(cur_do != NULL && cur_while != NULL);
 	 cur->add_successor(mem_ctx, cur_do);
 	 set_next_block(&cur, cur_while, ip);