RFR: JDK-8276983: Small fixes to DumpAllocStat::print_stats

[Thomas Stuefe]

When looking at CDS code in the context of Lilliput, I had to spend some time in DumpAllocStat::print(). I noticed two small things which can be fixed independently:

- the divide-by-zero check at lines 45ff is not needed, since `percent_of` does this already. It also can cause the asserts at the end of the function to fire wrongly.

- About those asserts: it makes sense to flush the debug message before scope end, otherwise, we won't see the debug message if the asserts fire. If they fire, the debug message would be helpful.

I also improved the assert message somewhat. I noticed that all sizes in this method are `int`, but left it that way because changing it would have too many ripple effects. I guess we won't get CDS archives beyond int_max any time soon.

Thanks, Thomas

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Commit messages:
 - Small fixes to DumpAllocStat::print_stats

Changes: https://git.openjdk.java.net/jdk/pull/6347/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6347&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276983
  Stats: 13 lines in 1 file changed: 3 ins; 8 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6347.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6347/head:pull/6347

PR: https://git.openjdk.java.net/jdk/pull/6347