RFR: JDK-8267481: Make sure table row has correct number of cells

[Hannes Wallnöfer]

This is a trivial change to throw a NPE or IllegalArgumentException if the number of cells passed to `addRow` does not match the number of column styles. (The non-nullness of `columnStyles` is already implied by other Table methods such as `toContent()`.)

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Commit messages:
 - JDK-8267481: Make sure table row has correct number of cells

Changes: https://git.openjdk.java.net/jdk/pull/4131/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4131&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267481
  Stats: 6 lines in 1 file changed: 3 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4131.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4131/head:pull/4131

PR: https://git.openjdk.java.net/jdk/pull/4131