PvtConstCompr: initialize the temperature vs viscosity table pointers in all constructors

this fixes opm-autodiff's "boprops_ad" unit test. thanks to
[at]atgeirr for pointing this out.

opm/core/props/pvt/PvtConstCompr.hpp

@@ -47,6 +47,8 @@ namespace Opm
     {
     public:
         PvtConstCompr()
+            : oilvisctTables_(0),
+              watvisctTables_(0)
         {}
 
         void initFromWater(Opm::DeckKeywordConstPtr pvtwKeyword)