RFR: 8073061: (fs) Files.copy(foo, bar, REPLACE_EXISTING) deletes bar even if foo is not readable

[Brian Burkhalter]

Check that the source is readable before deleting the destination.

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Commit messages:
 - 8073061: (fs) Files.copy(foo, bar, REPLACE_EXISTING) deletes bar even if foo is not readable

Changes: https://git.openjdk.org/jdk/pull/15501/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15501&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8073061
  Stats: 38 lines in 4 files changed: 33 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/15501.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15501/head:pull/15501

PR: https://git.openjdk.org/jdk/pull/15501