RFR: 8266018: Shenandoah: fix an incorrect assert

[wattsun110]

May I get reviews for this one-line change which fixes an incorrect assert?

thanks
watt

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Commit messages:
 - 8266018: Shenandoah: fix an incorrect assert

Changes: https://git.openjdk.java.net/jdk/pull/3710/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3710&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266018
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3710.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3710/head:pull/3710

PR: https://git.openjdk.java.net/jdk/pull/3710