Fix race condition with QProcess on OSX 10.7

Closing the forkfd descriptor before disabling the deathNotifier causes
a race condition in ~QProcess. This is the same underlying cause as
QTBUG-22789.

Change-Id: I1cbdedc567fdfa8d95d111827b7bf9994661ecc7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qprocess_unix.cpp

@@ -1101,6 +1101,9 @@ bool QProcessPrivate::waitForDeadChild()
     exitCode = info.si_status;
     crashed = info.si_code != CLD_EXITED;
 
+    delete deathNotifier;
+    deathNotifier = 0;
+
     qt_safe_close(forkfd);
     forkfd = -1; // Child is dead, don't try to kill it anymore