RFR: 8303475: potential null pointer dereference in filemap.cpp

[Calvin Cheung]

Please review this small patch which adds an assert in `SharedClassPathEntry::copy_from()` to ensure the input arg `SharedClassPathEntry* ent` is non null. This suppresses a warning from an internal static analysis tool.

Passed tiers 1 and 3 testing.

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

Commit messages:
 - 8303475: potential null pointer dereference in filemap.cpp

Changes: https://git.openjdk.org/jdk/pull/13708/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13708&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303475
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13708.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13708/head:pull/13708

PR: https://git.openjdk.org/jdk/pull/13708