bpo-34770: Fix a possible null pointer dereference in pyshellext.cpp (GH-9497)

The GlobalLock() call in UpdateDropDescription() was not checked for
failure.

https://bugs.python.org/issue34770
(cherry picked from commit f6c8007)

Co-authored-by: Zackery Spytz <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/Windows/2018-09-22-11-02-35.bpo-34770.4lEUOd.rst

@@ -0,0 +1 @@
+Fix a possible null pointer dereference in pyshellext.cpp.

PC/pyshellext.cpp

@@ -172,6 +172,11 @@ class PyShellExt : public RuntimeClass<
             return E_FAIL;
         }
         auto dd = (DROPDESCRIPTION*)GlobalLock(medium.hGlobal);
+        if (!dd) {
+            OutputDebugString(L"PyShellExt::UpdateDropDescription - failed to lock DROPDESCRIPTION hGlobal");
+            ReleaseStgMedium(&medium);
+            return E_FAIL;
+        }
         StringCchCopy(dd->szMessage, sizeof(dd->szMessage) / sizeof(dd->szMessage[0]), DRAG_MESSAGE);
         StringCchCopy(dd->szInsert, sizeof(dd->szInsert) / sizeof(dd->szInsert[0]), PathFindFileNameW(target));
         dd->type = DROPIMAGE_MOVE;