AthAllocators: Fix gcc11 warning.

Check the result of dynamic_cast before dereferencing.

This merge request affects 1 package:

• Control/AthAllocators

This merge request affects 1 file:

• Control/AthAllocators/AthAllocators/ArenaHandleBaseAllocT.icc

Adding @ssnyder as watcher

added Core master review-pending-level-1 labels

CI Result SUCCESS (hash dae3057f)

	Athena	AthSimulation	AthGeneration	AnalysisBase
externals
cmake
make
required tests
optional tests

Full details available on this CI monitor view
Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 0
AthGeneration: number of compilation errors 0, warnings 0
AnalysisBase: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 24745]

added review-approved label and removed review-pending-level-1 label

merged

mentioned in commit 5099a224

added sweep:ignore label