[TObjArray] Add GetEntriesUnsafe

GetEntriesFast is not as fast as it could be: it constructs and
destructs a TReadLockGuard, and might need to modify fLast.
GetEntriesUnsafe is a thread-unsafe version of GetEntriesFast
that side-steps these operations when possible.

core/cont/inc/TObjArray.h

@@ -64,6 +64,7 @@ public:
    Int_t            GetEntriesFast() const {
       return GetAbsLast() + 1;   //only OK when no gaps
    }
+   Int_t            GetEntriesUnsafe() const;
    Int_t            GetLast() const;
    TObject        **GetObjectRef() const { return fCont; };
    TObject        **GetObjectRef(const TObject *obj) const;

core/cont/src/TObjArray.cxx

@@ -554,6 +554,19 @@ Int_t TObjArray::GetAbsLast() const
    return fLast;
 }
 
+////////////////////////////////////////////////////////////////////////////////
+/// Return the number of objects in array (i.e. number of non-empty slots).
+/// This is a thread-unsafe version of GetEntriesFast. Use it only if sure
+/// it will not be invoked concurrently.
+
+Int_t TObjArray::GetEntriesUnsafe() const
+{
+   if (R__unlikely(fLast == -2))
+      return GetEntriesFast();
+   else
+      return fLast + 1;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 /// Return index of last object in array. Returns lowerBound-1 in case
 /// array is empty.