Address problem reported in ATLAS-7273

Triaged to likely static deletion order in G4ios, where use of std::cout
may result in accessing a no-longer present resource. Addressed upstream
in Geant4 in SIM-879.

Backport patch from SIM-879 from Geant4 to ATLAS' 10.6.3 version.

source/global/management/include/G4strstreambuf.icc

@@ -44,10 +44,6 @@ inline G4strstreambuf::G4strstreambuf()
 inline G4strstreambuf::~G4strstreambuf()
 ////////////////////////////////////////
 {
-  // flushing buffer...
-  // std::cout is used because destination object may not be alive.
-  if(count !=0) std::cout << buffer;
-
   delete [] buffer;
 }
 

source/global/management/src/G4ios.cc

@@ -78,6 +78,8 @@ void G4iosInitialization()
 
 void G4iosFinalization()
 {
+    _G4cout_p()->flush();
+    _G4cerr_p()->flush();
     delete _G4cout_p(); _G4cout_p() = &std::cout;
     delete _G4cerr_p(); _G4cerr_p() = &std::cerr;
     delete _G4coutbuf_p(); _G4coutbuf_p() = nullptr;
@@ -104,6 +106,9 @@ std::ostream G4cout(&G4coutbuf);
 std::ostream G4cerr(&G4cerrbuf);
 
 void G4iosInitialization() {}
-void G4iosFinalization() {}
+void G4iosFinalization() {
+    G4cout.flush();
+    G4cerr.flush();
+}
 
 #endif