[jdk17u-dev] RFR: 8303475: potential null pointer dereference in filemap.cpp

[Goetz Lindenmaier]

I backport this for parity with 17.0.18-oracle.

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Commit messages:
 - Backport e1196584d5c61e3a6baa3eb96801a518cd5dca64

Changes: https://git.openjdk.org/jdk17u-dev/pull/4199/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=4199&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/jdk17u-dev/pull/4199.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/4199/head:pull/4199

PR: https://git.openjdk.org/jdk17u-dev/pull/4199