Take a look at this case: Both Protoss and Zerg don't have the ability to gather resources anymore, Nexus and Hatchery are destroyed. Protoss still have a few base up. What's left of Zerg's base will eventually die because there's no more creep. The game detected stalemate even though the Protoss is set to win? Is this is a bug?
Here's a link to the replay: http://depositfiles.com/files/kzonxjxh4
Fast forward to the end and you'll see the Zerg's base is dying on it's own and Protoss should've won.
Best Answer
A stalemate is detected when no one does the following for 6 minutes (3 mins for a warning, then 3 mins for the stalemate).
(Source)
It sounds like all of these condition were met above, so I would say it is not a "bug."
As for the Zerg buildings dying, I believe buildings off creep lose health at 2 hp/second, and that some Zerg buildings have 1000 health, which means they can survive 8.3 minutes off creep. So it is possible that the remaining Zerg buildings are dying (but not yet dead) when the stalemate is declared.
Based on the above, I would say this is not a bug in the game, but an unforeseen possibility, that is not handled by the stalemate detection logic. Given how rare this situation is, I can see that.
You could post this on Blizzard's forums, or email them about it, and maybe people would be interested in changing the system to detect this.