stack-use-after-scope in treetreeplayertestUnit

[hahnjo]

After #10005 is merged, the test gtest-tree-treeplayer-test-treetreeplayertestUnit will fail due to a stack-use-after-scope:

434: READ of size 4 at 0x7f17de5323ac thread T0
434:     #0 0x7f17e571e7e6 in TObject::TestBit(unsigned int) const /home/jhahnfel/ROOT/src/core/base/inc/TObject.h:187:72
434:     #1 0x7f17e571e7e6 in TTree::~TTree() /home/jhahnfel/ROOT/src/tree/tree/src/TTree.cxx:1011:23
434:     #2 0x5a626a in TTreeReaderBasic_EntryListAndEntryRange_Test::TestBody() /home/jhahnfel/ROOT/src/tree/treeplayer/test/basic.cxx:407:1

[...]

434: Address 0x7f17de5323ac is located in stack of thread T0 at offset 940 in frame
434:     #0 0x5a4b3f in TTreeReaderBasic_EntryListAndEntryRange_Test::TestBody() /home/jhahnfel/ROOT/src/tree/treeplayer/test/basic.cxx:385

Note: There may be other problems in the test after this issue is fixed. Please check locally with an instrumented build that the test passes afterwards!