[BUGFIX] Don't modify variables in tree view

The lowlevel array browser tries to modify the array that
is displayed if it's an object.
Now that TYPO3_LOADED_EXT is a simulated array (an object
which implements array access) modifying the data leads to
a fatal error. Instead the iterated value is now modified.

Releases: 6.2
Resolves: #54449
Change-Id: Ib1d3eb5cc76a4180ea0891d88c16191cd16f36e2
Reviewed-on: https://review.typo3.org/27850
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters