RFR: 8300821: UB: Applying non-zero offset to non-null pointer 0xfffffffffffffffe produced null pointer

[Tobias Holenstein]

"UndefinedBehaviorSanitizer" (https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html) in Xcode running on `java --version` discovered  an Undefined Behavior. The reason is in the `next()` method https://github.com/openjdk/jdk/blob/040f5b55bd03bcc2209ece6eebf223ba1fabf824/src/hotspot/share/asm/codeBuffer.cpp#L798 

In ``RelocIterator::next()`` we get a nullpointer after `_current++`
https://github.com/openjdk/jdk/blob/040f5b55bd03bcc2209ece6eebf223ba1fabf824/src/hotspot/share/code/relocInfo.hpp#L612
But this is actually expected: In the constructor of the iterator `RelocIterator::RelocIterator` we have 
```c++
_current = cs->locs_start()-1;
_end     = cs->locs_end();

and in our case locs_start() and locs_end() are `null` - so `_current` is `null`-1. After `_current++` both `_end` and `_current` are `null`. Just after `_current++` we then check if `_current == _end` and return `false` (there is no next reloc info)

## Solution
We want to be able to turn on "UndefinedBehaviorSanitizer" and don't have false positives. So we add a check 
`cs->has_locs()` and only create the iterator if we have reloc info. 

Also added a sanity check in `RelocIterator::RelocIterator` that checks that either both `_current` and `_end` are null or both are not null.

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

Commit messages:
 - UB: Applying non-zero offset to non-null pointer 0xfffffffffffffffe produced null pointer

Changes: https://git.openjdk.org/jdk/pull/12854/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12854&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300821
  Stats: 4 lines in 2 files changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12854.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12854/head:pull/12854

PR: https://git.openjdk.org/jdk/pull/12854