RFR: 8268361: Fix the infinite loop in next_line

[UncleNine]

If the /proc/stat mount point is changed in container environment, the while loop may lead to 100% cpu usage.

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Commit messages:
 - Fix the dead loop.\nSummary: If the /proc/stat mount point is changed in container environment, the while loop may lead to 100% cpu usage.

Changes: https://git.openjdk.java.net/jdk/pull/4378/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4378&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268361
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4378.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4378/head:pull/4378

PR: https://git.openjdk.java.net/jdk/pull/4378