Multiple solutions are possible for this problem. Currently, you are creating decals of a standard size, which gives problems of course.

Solution 1
Update the actual texture, don't use decals. Write your decal to the texture instead.

Solution 2
Perform multitexturing on every object that is shootable. Create a new texture with your decals in it, wrap it onto the building.
This solution is a memory eater, it creates textures for every object

Solution 3
Do a check of your decal when adding it to the wall. You could think of shooting rays from the back of the decal again. Or do a box intersect. The intersection points will let you clip your decal texture.
Yes this is a hard one, but gives you more freedom in your decals

The cheapest solution is texture manipulation. (But I think you will need a renderTexture for that?) It will result in the same amount of draw calls, where your decals create 2 extra triangles per decal (100000 bullets ouch).

Update
I found this little project on the forum: http://forum.unity3d.com/threads/2562-blood-splatter

Awnser:

don't shoot to close at the corners haha

no just kidding...

what you could do (some sort of workaround) is create an non-collidable invisible wall right next to the corner that will overlap the decal.

just a thought :)