ShellPkg/Ifconfig6: Address ASSERT because of a missing NULL check

ShellPkg/Ifconfig6: Address ASSERT because of a missing NULL check

When we issue 'ifconfig6 -s <interface> auto' system hangs with
an ASSERT in StrLen. in IfConfig6SetInterfaceInfo, for 'auto' case
we added checks to rule out the invalid inputs like 'host', 'gw'
and 'dns'. To parse through this, we do a VarArg = VarArg->Next but
we dont check new VarArg before calling StrCmp. Fix with a check
in this patch.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hegde Nagaraj P <nagaraj-p.hegde@hpe.com>
Reviewed-by: Zhang Lubo <lubo.zhang@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Sriram Subramanian <sriram-s@hpe.com>