BlackoilPropsAdFromDeck: don't rely on undefined behaviour when initializing the fluid system

in C++ references may be assumed to be non-null. thanks to
[at]atgeirr for pointing this out.

opm/autodiff/BlackoilPropsAdFromDeck.cpp

@@ -158,7 +158,7 @@ BlackoilPropsAdFromDeck::BlackoilPropsAdFromDeck(const BlackoilPropsAdFromDeck&
 
         phase_usage_ = phaseUsageFromDeck(deck);
 
-        if (! (&FluidSystem::oilPvt()) ) {
+        if (!FluidSystem::isInitialized()) {
             // make sure that we don't initialize the fluid system twice
             FluidSystem::initFromDeck(deck, eclState);
         }