Another incomplete fix for the NPE bugs in ParallelReader.java [LUCENE-3781]

The fix revision 407851 was aimed to remove an NPE bug ("fix NPE and deletion bugs in ParallelReader: LUCENE-561") on the value of "termDocs" in the methods "next", "read", "skipTo", "close" of the file "/lucene/java/trunk/src/java/org/apache/lucene/index/ParallelReader.java
" , but it is incomplete.
Since the value "termDocs" could be null during the runtime execution, its value should also be null-checked before being dereferenced in other methods.

The buggy code locations the same fix needs to be applied at are as bellows:

Line 574, 575 of the methods "doc()" , and "freq":

public int doc() { return termDocs.doc(); }
public int freq() { return termDocs.freq(); }

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Migrated from [LUCENE-3781](https://issues.apache.org/jira/browse/LUCENE-3781) by Guangtai Liang

Start by locating lucene/java/trunk/src/java/org/apache/lucene/index/ParallelReader.java and checking whether this historical path still exists. Inspect the doc() and freq() methods around lines 574-575 and compare them with the existing termDocs handling in next, read, skipTo, and close; done means those methods safely handle a null termDocs value.

Written by the indexing model from the issue text.