git-p4: check for access to remote host earlier

Check we can talk to the remote host before starting the git-fastimport
subchild.

Otherwise we fail to connect, and then exit, leaving git-fastimport
still running since we did not wait() for it.

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

git-p4.py

@@ -3826,6 +3826,8 @@ class P4Sync(Command, P4UserMap):
                     b = b[len(self.projectName):]
                 self.createdBranches.add(b)
 
+        p4_check_access()
+
         self.openStreams()
 
         err = None