Snape probably was cursed (and told Voldemort so)
We know that Mad-Eye Moody (a wizard of considerable skill) placed a spell on Grimmauld Place meant to prevent Snape from revealing its location:
“But surely Snape will have told the Death Eaters the address by now?”
asked Harry.
“Well, Mad-Eye set up a couple of curses against Snape in case he
turns up there again. We hope they’ll be strong enough both to keep
him out and to bind his tongue if he tries to talk about the place,
but we can’t be sure. It would have been insane to keep using the
place as headquarters now that its protection has become so shaky.”
—Harry Potter and the Deathly Hallows
Presumably there were spells that did more than literally "bind his tongue," since we know from Order of the Phoenix that the secret can be revealed through writing.
As long as Snape showed up, he should have been both prevented from entering and from speaking its location.
We know he went to Grimmauld Place:
And next, Snape was kneeling in Sirius’s old bedroom. Tears were
dripping from the end of his hooked nose as he read the old letter
from Lily.
—Harry Potter and the Deathly Hallows
Of course, the spells clearly did not succeed in keeping him out, since he managed to get to Sirius's bedroom. However, there would be no reason for Snape not to deliberately trigger the Tongue-Tying Curse. By doing so, he could ensure that there was no way he could reveal the information to Voldemort, even under torture, the Imperius Curse, Veritaserum, or some other form of compulsion.
Snape lied to Voldemort
Snape was one of the best Occlumens around, capable of lying to Lord Voldemort's face without getting caught.
“But he’s a very good Occlumens, isn’t he, sir?” said Harry, whose
voice was shaking with the effort of keeping it steady. “And isn’t
Voldemort convinced that Snape’s on his side, even now?
—Harry Potter and the Half-Blood Prince
What would Snape have claimed? Why, something very close to the truth. He would claim that he had been to Grimmauld Place and been the victim of Moody's spell, which Voldemort could possibly confirm. All he would need to conceal would be that he had done this intentionally. If Moody's curse had been insufficient, he could have placed his own, or perhaps even lied to Voldemort about the scope or nature of the curse. Even unto the end, Voldemort trusted Snape (understandable, given what Snape had sacrificed for him). Further, his supreme confidence in his own powers of Legilimency would have lulled him into believing any information gleaned from Snape's mind.