Fix dangling reference in InjectionTargetCalculator

Store injection_phase by value instead of by reference to prevent
dangling reference when the calculator is returned from a function.

The previous code stored a const reference to the Phase parameter,
which works when the calculator is constructed and used within the
same scope (e.g., in GroupStateHelper::checkGroupConstraintsInj).
However, when the calculator is returned from a e.g a function
the temporary Phase value passed to the constructor is destroyed, leaving
a dangling reference that can cause undefined behavior.

opm/simulators/wells/TargetCalculator.hpp

@@ -98,7 +98,7 @@ private:
     const GroupStateHelperType& groupStateHelper_;
     const std::vector<Scalar>& resv_coeff_;
     const Group& group_;
-    const Phase& injection_phase_;
+    const Phase injection_phase_;
     Group::InjectionCMode cmode_;
     int pos_;
     GuideRateModel::Target target_;