RFR: 8264624: change the guarantee() calls added by JDK-8264123 to assert() calls

[Daniel D.Daugherty]

A trivial fix to change the guarantee() calls added by JDK-8264123 to assert() calls.
See the bug report for the details.

Tested with Mach5 Tier1.

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Commit messages:
 - 8264624: change the guarantee() calls added by JDK-8264123 to assert() calls

Changes: https://git.openjdk.java.net/jdk/pull/4236/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4236&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264624
  Stats: 17 lines in 2 files changed: 4 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4236.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4236/head:pull/4236

PR: https://git.openjdk.java.net/jdk/pull/4236