MdePkg: check Length para before use in DevPathToTextUsbWWID

MdePkg: check Length para before use in DevPathToTextUsbWWID

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1336

In function DevPathToTextUsbWWID, the Length parameter is used
without check. This patch is to add check before using it.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>