RFR: 8272541: Incorrect overflow test in Toom-Cook branch of BigInteger multiplication

[Brian Burkhalter]

Please consider this change which would modify a conditional `a + b > c` where the left side variables are `int`s and the right side is `(long)Integer.MAX_VALUE + 1`. The change is to cast the left side variables to `long`s.

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Commit messages:
 - 8272541: Incorrect overflow test in Toom-Cook branch of BigInteger multiplication

Changes: https://git.openjdk.java.net/jdk/pull/5130/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5130&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272541
  Stats: 30 lines in 2 files changed: 18 ins; 3 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5130.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5130/head:pull/5130

PR: https://git.openjdk.java.net/jdk/pull/5130