MdeModulePkg: Do not dump NULL padding resource descriptor

MdeModulePkg: Do not dump NULL padding resource descriptor

Add a check for ResourcePaddingDescriptors being a valid pointer in
DumpPpbPaddingResource() to prevent looping on memory not owned by
PciBusDxe.  The ResourcePaddingDescriptors is initialized to NULL
when the PCI_IO_DEVICE structure is allocated and remains NULL if
no PCI hot plug controllers are present.  This issue is only
observed when DEBUG_CODE() macros are enabled and was introduced
by the following patch:

  [edk2] [Patch] MdeModulePkg: Fix a PciBusDxe hot plug bug
  SVN revsion 18658

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Kinney Michael <michael.d.kinney@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18696 6f19259b-4bc3-4df7-8a09-765794883524