RFR: JDK-8268582: javadoc throws NPE with --ignore-source-errors option

[Hannes Wallnöfer]

Please review a simple fix for a javadoc NPE when using the undocumented `--ignore-source-errors` option with an non-existing type.

Note that this just fixes the particular error that occurs when looking for preview API use in invalid types. It is possible that other source errors trigger other errors in javadoc, but I don't think it is worth the time to go and actively look for them.

In particular, the NPE that is documented by the stack trace in the JBS issue was different from the one fixed here and does not occur anymore, and the one fixed here did not yet exist in JDK 11.

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Commit messages:
 - JDK-8268582: javadoc throws NPE with JDK 11

Changes: https://git.openjdk.java.net/jdk/pull/6506/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6506&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268582
  Stats: 21 lines in 2 files changed: 16 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6506.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6506/head:pull/6506

PR: https://git.openjdk.java.net/jdk/pull/6506