RFR: 8272576: G1: Use more accurate integer type for collection set length

[Albert Mingkun Yang]

Simple change of using smaller integer type, `size_t` to `uint`. Also moved the bound-check assertion before array access.

Test: hotspot_gc

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Commit messages:
 - uint-length

Changes: https://git.openjdk.java.net/jdk/pull/5148/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5148&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272576
  Stats: 10 lines in 2 files changed: 2 ins; 2 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5148.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5148/head:pull/5148

PR: https://git.openjdk.java.net/jdk/pull/5148