NetworkPkg/Ip6Dxe: Validate source data record length

NetworkPkg/Ip6Dxe: Validate source data record length

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2273

Ip6ConfigReadConfigData() reads configuration data from a UEFI variable
and copies the data to another buffer. This change checks that the
length
of the data record being copied does not exceed the size of the source
UEFI variable data buffer.

If the size is exceeded, this change follows existing logic to treat the
variable as corrupted and deletes the variable so it will be set again.

Cc: Siyuan Fu <siyuan.fu@intel.com>
Cc: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>
Reviewed-by: Maciej Rabeda <maciej.rabeda@linux.intel.com>