That's a hell of a good idea. As somebody pointed out, it might not stop you from corrupting neighboring items in an array or structure, but it would let you find the size of an array allocated by itself, AND, it would stop you from corrupting the heap itself!
If you're concerned about corrupting the heap, use an allocator hardened against heap corruption. The default WinAPI allocator, even for optimized production code, is hardened that way. Userland code doesn't need to do anything to get the feature, which is as it should be, because people who write userland code don't know enough to defend the heap against memory corruption.
