ArmPkg/BdsLib: Fix DHCP4 configuration

ArmPkg/BdsLib: Fix DHCP4 configuration

Fix DHCP4 configuration as part of TFTP boot process to get the IP address of
the gateway and the subnet mask from the DHCP server.
Fix a memory leak when the download from the TFTP server fails as well.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ronald Cron <Ronald.Cron@arm.com>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16582 6f19259b-4bc3-4df7-8a09-765794883524