IntelFrameworkPkg FrameworkUefiLib: Fix ASSERT in CatVSPrint

IntelFrameworkPkg FrameworkUefiLib: Fix ASSERT in CatVSPrint

This commit will resolve issue brought by r17740.

BufferToReturn = AllocateCopyPool(SizeRequired, String);

The above using of AllocateCopyPool() will cause ASSERT if 'String' is
NULL. Therefore, proper check for 'String' is needed.

The above using of AllocateCopyPool() will read contents out of the scope
of 'String'. Potential risk for 'String' allocated at the boundary of
memory region.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17906 6f19259b-4bc3-4df7-8a09-765794883524