8368997: AIX allows reading from address zero which leads to several ubsan findings

Reviewed-by: mdoerr, mbaesken

Author: Joachim Kern

src/hotspot/os/posix/safefetch_sigjmp.cpp

@@ -67,6 +67,14 @@ ATTRIBUTE_NO_ASAN static bool _SafeFetchXX_internal(const T *adr, T* result) {
 
  T n = 0;
 
+#ifdef AIX
+ // AIX allows reading from nullptr without signalling
+ if (adr == nullptr) {
+ *result = 0;
+ return false;
+ }
+#endif
+
  // Set up a jump buffer. Anchor its pointer in TLS. Then read from the unsafe address.
  // If that address was invalid, we fault, and in the signal handler we will jump back
  // to the jump point.