Fix setting inheritance on None-fd

When gnuplotlib is called from an environment that changes sys.stdout,
then self.fdDupSTDOUT is None. However, we still try to make the fd
inheritable, which fails.

This fix only makes the duplicated fd inheritable if it in fact was able
to be duplicated.

Author: mbr0wn

gnuplotlib.py

@@ -1502,10 +1502,11 @@ def _startgnuplot(self):
         # would happen by default, but in python3 I need to do this extra thing
         # for some reason. And it's a new thing that didn't exist in python2, so
         # I need to explicitly allow this to fail in python2
-        try:
-            os.set_inheritable(self.fdDupSTDOUT, True)
-        except AttributeError:
-            pass
+        if self.fdDupSTDOUT:
+            try:
+                os.set_inheritable(self.fdDupSTDOUT, True)
+            except AttributeError:
+                pass
 
         self.gnuplotProcess = \
             subprocess.Popen(cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE,