RFR: 8282573: ByteBufferTest.java: replace endless recursion with RuntimeException in void ck(double x, double y)

[Emanuel Peter]

There are two functions that check for equality.
void ck(long x, long y): already throws a RuntimeException if we observe inequality
void ck(double x, double y): called itself, leading to endless recursion and a StackOverflowError.

Instead of the recursion, I now also throw a RuntimeException.

This can currently be triggered with
`path-to-jtreg/jtreg -va -s -jdk:jdk-path -javaoptions:"-XX:+UnlockDiagnosticVMOptions -XX:-TieredCompilation -XX:+StressGCM -XX:+OptoScheduling -XX:StressSeed=293843391" test/hotspot/jtreg/compiler/intrinsics/unsafe/HeapByteBufferTest.java `
(see bug [JDK-8282555](https://bugs.openjdk.java.net/browse/JDK-8282555))

This way I could verify that instead of StackOverflowError we now get RuntimeException, as desired.
Ran some basic tests to ensure I didn't break things.

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Commit messages:
 - missed a line
 - 8282573: ByteBufferTest.java: replace endless recursion with RuntimeException in void ck(double x, double y)

Changes: https://git.openjdk.java.net/jdk/pull/7674/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7674&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282573
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7674.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7674/head:pull/7674

PR: https://git.openjdk.java.net/jdk/pull/7674