git-p4: make closeStreams() idempotent

Ensure that we can safely call self.closeStreams() multiple times, and
can also call it even if there is no git fast-import stream at all.

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -3549,11 +3549,14 @@ def openStreams(self):
         self.gitError = self.importProcess.stderr
 
     def closeStreams(self):
+        if self.gitStream is None:
+            return
         self.gitStream.close()
         if self.importProcess.wait() != 0:
             die("fast-import failed: %s" % self.gitError.read())
         self.gitOutput.close()
         self.gitError.close()
+        self.gitStream = None
 
     def run(self, args):
         if self.importIntoRemotes: