FourMom: Avoid conversion of null pointers to references.

Avoid a conversion of a null pointer to a reference, caught by ubsan.

Event/FourMom/FourMom/DeepCopyPointer.h

@@ -35,6 +35,9 @@ public:
     return *this;
   }
 
+        T* get()       { return m_theData; }
+  const T* get() const { return m_theData; }
+
   T&       operator*()       { return *m_theData;}
   const T& operator*() const { return *m_theData;}
 

Event/FourMom/FourMom/Lib/P4ImplEEtaPhiM.h

@@ -215,7 +215,7 @@ inline double P4ImplEEtaPhiM::e() const
 
 inline const I4MomentumError* P4ImplEEtaPhiM::errors() const
 {
-  return &(*m_error);
+  return m_error.get();
 }
 
 inline void P4ImplEEtaPhiM::setE( const double theE )

Event/FourMom/FourMom/Lib/P4ImplIPtCotThPhiM.h

@@ -228,8 +228,7 @@ inline double P4ImplIPtCotThPhiM::cotTh() const
 
 inline const I4MomentumError* P4ImplIPtCotThPhiM::errors() const
 {
-  // check if the pointer is empty before dereferencing
-  return (m_error ? &(*m_error) : 0);
+  return m_error.get();
 }
 
 // setters

Event/FourMom/FourMom/Lib/P4ImplPtEtaPhiM.h

@@ -212,7 +212,7 @@ inline double P4ImplPtEtaPhiM::pt() const
 inline const I4MomentumError* P4ImplPtEtaPhiM::errors() const
 {
   // check if the pointer is empty before dereferencing
-  return (m_error ? &(*m_error) : 0);
+  return m_error.get();
 }
 
 // setters

Event/FourMom/FourMom/Lib/P4ImplPxPyPzE.h

@@ -219,7 +219,7 @@ inline double P4ImplPxPyPzE::e() const
 inline const I4MomentumError* P4ImplPxPyPzE::errors() const
 {
   // check if the pointer is empty before dereferencing
-  return (m_error ? &(*m_error) : 0);
+  return m_error.get();
 }
 
 inline void P4ImplPxPyPzE::setPx( const double px )