Harry wished, with everything that he had, that he could guarantee her it wouldn't happen. He didn't think he could, though. There was no guarantee for him that he'd survive any of this, especially if the end was as near as he thought it was. At least, the end of his fight against Voldemort, in which one of them would destroy the other. He didn't know what would happen after that, especially if he was the one who was destroyed.
"I don't want that to happen either," he said quietly, taking her hand. "But that could happen no matter where you are. I'll be careful." It had gotten easier to bear, the precautions he needed to take to keep himself safe, after he'd realized that there were people outside of Ron and Hermione who still wanted him to live. Most importantly, Ginny. But he didn't know how to say that, didn't know if he should.