RFR: 8280459: Suspicious integer division in Hashtable.readHashtable

[Aleksey Shipilev]

Found by Sonar. See details in the bug.

Additional testing:
 - [x] Linux x86_64 fastdebug `tier1`
 - [x] Linux x86_64 fastdebug `java/util/Hashtable`

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/7178/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7178&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280459
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7178.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7178/head:pull/7178

PR: https://git.openjdk.java.net/jdk/pull/7178