RFR: 8362846: Windows error reporting for dll_load doesn't check for a null buffer

[David Holmes]

Please review this simple fix for some missing null checks in `os::dll_load`. The primary issue handles Windows, while the additional is for Linux/BSD.

Testing:
- new gtest
- tiers 1-3

Thanks

-------------

Commit messages:
 - 8362846: Windows error reporting for dll_load doesn't check for a null buffer
 - 8362954: Missing error buffer null check in os::dll_load on Linux/BSD

Changes: https://git.openjdk.org/jdk/pull/26420/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26420&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8362846
  Stats: 23 lines in 4 files changed: 22 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26420.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26420/head:pull/26420

PR: https://git.openjdk.org/jdk/pull/26420