ShellPkg/App: Fix memory leak and save resources.

ShellPkg/App: Fix memory leak and save resources.

1) RunSplitCommand() allocates the initial SplitStdOut via
   CreateFileInterfaceMem(). Free SplitStdIn after the swap to fix
   the memory leak.

2) In RunSplitCommand(), SplitStdOut is checked for equality with
   StdIn. This cannot happen due to the if-check within the swap.
   Hence remove it.

3) UefiMain() doesn't free SplitList. Delete all list entries and
   reinitialize the list when in DEBUG. This does not include the
   CreateFileInterfaceMem()-allocated SplitStd mentioned in 1), so
   keep the ASSERT() until resolved.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Marvin Haeuser <Marvin.Haeuser@outlook.com>
Reviewed-by: Qiu Shumin <shumin.qiu@intel.com>