SourceLevelDebugPkg DebugCommLibUsb3: Address NULL ptr dereference case

SourceLevelDebugPkg DebugCommLibUsb3: Address NULL ptr dereference case

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2051

Original code GetConfigTable for AddrPtr first, if failed,
InstalConfigTable with allocated AddrPtr. So the AddrPtr
should not be NULL and the NULL pointer dereference case
should be false positive. This patch is just to address
the case raised from static analysis.

Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>