RFR: 8278049: G1: add precondition to set_remainder_to_point_to_start

[Albert Mingkun Yang]

Simple change of converting always-false conditions to assertions.

Test: hotspot_gc

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Commit messages:
 - bot-precondition

Changes: https://git.openjdk.java.net/jdk/pull/6630/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6630&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278049
  Stats: 11 lines in 1 file changed: 0 ins; 9 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6630.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6630/head:pull/6630

PR: https://git.openjdk.java.net/jdk/pull/6630