Only deref a reserved gateway session if we actually reserved one

Its perfectly acceptable to have a gateway session unreserved when we go to
first allocate one.  Unreffing the reserved gateway session - when its NULL -
will result in an assertion error.

This problem was caught by the Asterisk Test Suite (once we had enough of the
debugging flags enabled)
........

Merged revisions 375797 from http://svn.asterisk.org/svn/asterisk/branches/10
........

Merged revisions 375798 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@375799 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_fax.c

@@ -2583,7 +2583,9 @@ static int fax_gateway_start(struct fax_gateway *gateway, struct ast_fax_session
 	}
 	/* release the reference for the reserved session and replace it with
 	 * the real session */
-	ao2_ref(gateway->s, -1);
+	if (gateway->s) {
+		ao2_ref(gateway->s, -1);
+	}
 	gateway->s = s;
 	gateway->token = NULL;