ArmPkg: Fixed GetEnvironmentVariable() when the UEFI Variable did not exist

ArmPkg: Fixed GetEnvironmentVariable() when the UEFI Variable did not exist

The function was allocating a buffer for the read value from the UEFI Variable.
But it was returning the pointer of the default value when the variable was
not present.
It could cause error when the default value and the returned value were free
when these addresses were the same (double FreePool on the same address).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15427 6f19259b-4bc3-4df7-8a09-765794883524