RFR: 8312976: MatchResult produces StringIndexOutOfBoundsException for groups outside match

[Raffaello Giulietti]

Fixes a bug showing up after [JDK-8132995](https://bugs.openjdk.org/browse/JDK-8132995) when there are capturing groups outside the match.

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

Commit messages:
 - 8312976: MatchResult produces StringIndexOutOfBoundsException for groups outside match

Changes: https://git.openjdk.org/jdk/pull/15053/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15053&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312976
  Stats: 75 lines in 2 files changed: 67 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/15053.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15053/head:pull/15053

PR: https://git.openjdk.org/jdk/pull/15053