If you're looking for a solvent to store your heroin safely in for an unknown amount of time...I don't think there is one.
Like sekio has already pointed out, using bacteriostatic water will break the heroin down(into morphine I believe) fairly quickly.
Also...how do you intend to use this solution? IV? Because if you find a solvent that ISN'T water(won't break down your heroin into morphine), yet you still plan on using your solution for intravenous purposes....be very careful what you use as a solvent...as MANY of your options would wreak havoc on your veins.
...but if your plan is to make a solution for an intranasal spray, then the answers/help you receive from members of BL would likely change accordingly.