RFR: 8319882: SequenceLayout::toString throws ArithmeticException

[Per Minborg]

This PR proposes to fix a problem where a sequence layout contains an element of `byteSize()` zero.

-------------

Commit messages:
 - Fix toString problem

Changes: https://git.openjdk.org/jdk/pull/16599/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16599&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319882
  Stats: 16 lines in 2 files changed: 15 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16599.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16599/head:pull/16599

PR: https://git.openjdk.org/jdk/pull/16599