[SPARK-7736] [CORE] Fix a race introduced in PythonRunner.

The fix for SPARK-7736 introduced a race where a port value of "-1"
could be passed down to the pyspark process, causing it to fail to
connect back to the JVM. This change adds code to fix that race.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #8258 from vanzin/SPARK-7736.