Bug 1163452 - Only register stderr if it differs from stdout. r=glandium

Under certain Windows shells, the handle for stderr may be the same as
the handle for stdout.  Avoid crashing in this case.

Differential Revision: https://phabricator.services.mozilla.com/D150517

xpcom/build/PoisonIOInterposerWin.cpp

@@ -465,7 +465,9 @@ void InitPoisonIOInterposer() {
 
   // Stdout and Stderr are OK.
   MozillaRegisterDebugFD(1);
-  MozillaRegisterDebugFD(2);
+  if (::GetStdHandle(STD_OUTPUT_HANDLE) != ::GetStdHandle(STD_ERROR_HANDLE)) {
+    MozillaRegisterDebugFD(2);
+  }
 
 #ifdef MOZ_REPLACE_MALLOC
   // The contract with InitDebugFd is that the given registry can be used