MdePkg BaseUefiDecompressLib: Fix UEFI Decompression logic issue

MdePkg BaseUefiDecompressLib: Fix UEFI Decompression logic issue

https://bugzilla.tianocore.org/show_bug.cgi?id=1317

This is a regression issue caused by 2ec7953d49677142c5f7552e9e3d96fb406ba0c4.
In Decode() function, once mOutBuf is fully filled, Decode() should return.
Current logic misses the checker of mOutBuf after while() loop.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Liming Gao <liming.gao@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>