RFR[XS]: 8238961 Assertion failure in new field layout code when ContendedPaddingWidth == 0.

[Frederic Parain]

Please review this very small fix in field layout code.

The issue is that option ContendedPaddingWidth can be set to zero,
causing an assertion failure when trying to create a padding block
of size zero. One site inserting padding blocks was protected against
zero padding width, but another site was not. This fix makes the
second site robust against this case.

CR: https://bugs.openjdk.java.net/browse/JDK-8238961
Webrev: http://cr.openjdk.java.net/~fparain/8238961/webrev.00/index.html

Testing:
  - tests test/hotspot/jtreg/runtime/contended manually run with ContendedPaddingWidth
    forced to zero (tests fails because they are not designed to support the case
    where the padding width is zero, but the VM doesn’t crash anymore because of the
    assert)
  - tier 1-3

Thank you,

Fred