ArmPkg/BdsLib: Prevent memory leak whith TFTP

ArmPkg/BdsLib: Prevent memory leak whith TFTP

In some case, the size of the downloaded TFTP image cannot be known.
An arbitrary larger buffer is allocated to receive the image.
We need to make sure when we free the buffer we free the size
of the allocated buffer and not the size of the actual image.

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@15609 6f19259b-4bc3-4df7-8a09-765794883524