use signalstatus if exit status is undefined

fixes failing test_exit_code_signal on OS X

IPython/utils/_process_posix.py

@@ -188,6 +188,13 b' class ProcessHandler(object):'
         # We follow the subprocess pattern, returning either the exit status
         # as a positive number, or the terminating signal as a negative
         # number. sh returns 128+n for signals
+        if child.exitstatus is None:
+            # on WIFSIGNALED, pexpect sets signalstatus, leaving exitstatus=None
+            if child.signalstatus is None:
+                # this condition may never occur,
+                # but let's be certain we always return an integer.
+                return 0
+            return -child.signalstatus
         if child.exitstatus > 128:
             return -(child.exitstatus - 128)
         return child.exitstatus