[jdk15u-dev] RFR: 8268361: Fix the infinite loop in next_line

[Roman Marchenko]

Bakport of JDK-8268361 to 15u.
It needs to be ported due to similar problems as 17u has.
Fix applies cleanly. Tests tier1 passed after the fix without regression.

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Commit messages:
 - Backport 72672277e4dddf8e72f1c705cd5f57de40745635

Changes: https://git.openjdk.org/jdk15u-dev/pull/260/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk15u-dev&pr=260&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8268361
  Stats: 15 lines in 1 file changed: 11 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk15u-dev/pull/260.diff
  Fetch: git fetch https://git.openjdk.org/jdk15u-dev pull/260/head:pull/260

PR: https://git.openjdk.org/jdk15u-dev/pull/260