RFR: 8314148: Fix variable scope in SunMSCAPI

[Daniel Jeliński]

Please review this patch that fixes the scope of 2 local variables to make sure they are in scope when they are used.

The patch additionally fixes one `c++11-narrowing` Clang warning by explicitly casting string size (`size_t`) to `DWORD`.

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Commit messages:
 - Fix variable scope

Changes: https://git.openjdk.org/jdk/pull/15242/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15242&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314148
  Stats: 9 lines in 3 files changed: 2 ins; 2 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/15242.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15242/head:pull/15242

PR: https://git.openjdk.org/jdk/pull/15242