RFR: 8255466: C2 crashes at ciObject::get_oop() const+0x0

[Vladimir Kozlov]

Graal testing hit this issue with product VM. Tom R. suggested that it could be the case of reflective unsafe static field access that would eventually be optimized away because the Class is null:
`if (staticFieldBase != null) {
  return Unsafe.getInt(staticFieldBase, Unsafe.staticFieldOffset(field));
}`

I suggest to replace assert with runtime check. Note, `o` value is assigned to `_const_oop` so semantically new code is the same except additional runtime check.

I also noticed that const_oop is accessed without check for NULL in new Vector API code. I added check there too.

Passed tier1-3 testing.

-------------

Commit messages:
 - 8255466: C2 crashes at ciObject::get_oop() const+0x0

Changes: https://git.openjdk.java.net/jdk/pull/890/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=890&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255466
  Stats: 9 lines in 2 files changed: 5 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/890.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/890/head:pull/890

PR: https://git.openjdk.java.net/jdk/pull/890