RFR: JDK-8319704: LogTagSet::set_output_level() should not accept NULL as LogOutput

[Thomas Stuefe]

Trivial change to add an assert.

See [JDK-8319104](https://bugs.openjdk.org/browse/JDK-8319104)

`LogTagSet::set_output_level()` and `LogOutputList::set_output_level` should not accept NULL as LogOutput* argument.

They do, currently, which leads to delayed crashes later since the NULL output is registered in the output list.

That triggered [JDK-8319104](https://bugs.openjdk.org/browse/JDK-8319104), where - due to a fluke in C++ initialization order and a mislabeling of LogTagSet tests as "TEST", without "_VM" - `LogTagSet::set_output_level()` was called with `LogConfiguration::StdoutLog` which was still uninitialized (null). 

An assert would have saved investigation time.

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Commit messages:
 - JDK-8319704-LogTagSet-set_output_level-should-not-accept-NULL-as-LogOutput

Changes: https://git.openjdk.org/jdk/pull/16555/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16555&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319704
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16555.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16555/head:pull/16555

PR: https://git.openjdk.org/jdk/pull/16555