Checking for null objects...

I've run into a strange problem when checking to see if a pointer refers to a null object. I have added some code to Projectile, and from onCollision, here's a fragment:

DecalData* decal = mCurrTick > mDataBlock->armingDelay ?
				mDataBlock->decals[idx] :
				mDataBlock->bounceDecals[idx];
			if(decal != NULL)
			{
				DecalManager *decalMngr = gClientSceneGraph->getCurrentDecalManager();
				if(decalMngr)
					decalMngr->addDecal(hitPosition, hitNormal, decal);
			}

The integer idx is defined and set beforehand. Now, it's entirely possible that idx is an illegal value. Shouldn't the if block catch this? In my case, idx was being set to -1, and the if evaluated as true, allowing the decal manager to try to create a decal with rubbish for a datablock.

I strongly suspect this is just my ignorance of C++ showing through - I've never been formally instructed, I've just learned by reading code.
I guess the simplest solution is to just make sure idx is a legal value, right?

EDIT: I tried something I'd seen done elsewhere. The if expression I changed to:

if(bool(decal))

Now the function works the way it should.
What's the difference between these two methods?