GCC 12 fails to build zReferenceProcessor.cpp

When compiling with GCC 12.1.1 (current in Fedora rawhide), the following warning-as-error is produced and breaks the build by default:

```
/home/test/shipilev-jdk/src/hotspot/share/gc/z/zReferenceProcessor.cpp: In member function 'oopDesc* ZReferenceProcessor::drop(oop, ReferenceType)':
/home/test/shipilev-jdk/src/hotspot/share/gc/z/zReferenceProcessor.cpp:270:22: error: '%s' directive argument is null [-Werror=format-overflow=]
  270 | log_trace(gc, ref)("Dropped Reference: " PTR_FORMAT " (%s)", p2i(reference), reference_type_name(type));
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

The problem is over-zealous compiler seeing `NULL` from `ShouldNotReachHere()` block, and complaining about it. In this particular case, we can dodge this by returning a more reasonable constant on failure path.

(This thing should be more reasonably handled when `ShouldNotReachHere()` is somehow `nonreturn`-ed, but Hotspot style doc is still undecided on this).