RFR: 8309847: FrameForm and RegisterForm constructors should initialize all members

[Vladimir Petko]

This PR fixes missing constructor initialisations in formsopt.cpp.
This PR does not implement [CppCoreGuidelines#Rc-in-class-initializer](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-in-class-initializer ) to keep the style consistent and minimise changes.

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Commit messages:
 - 8309847: add missing member initialisations

Changes: https://git.openjdk.org/jdk/pull/14435/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14435&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309847
  Stats: 11 lines in 1 file changed: 9 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14435.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14435/head:pull/14435

PR: https://git.openjdk.org/jdk/pull/14435