NetworkPkg/TlsDxe: Fix failure to process multiple TLS records.

NetworkPkg/TlsDxe: Fix failure to process multiple TLS records.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1290.

Current implementation failed to parse the multiple TLS record
messages due to the incorrect pointer of TLS record header. This
patch is to resolve that problem.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>