8008560: [parfait] Null pointer deference in hotspot/src/os_cpu/bsd_x86/vm/os_bsd_x86.cpp

Summary: add null pointer check in signal handler
Reviewed-by: kvn

src/os_cpu/bsd_x86/vm/os_bsd_x86.cpp

@@ -516,7 +516,7 @@ JVM_handle_bsd_signal(int sig,
         // here if the underlying file has been truncated.
         // Do not crash the VM in such a case.
         CodeBlob* cb = CodeCache::find_blob_unsafe(pc);
-        nmethod* nm = cb->is_nmethod() ? (nmethod*)cb : NULL;
+        nmethod* nm = (cb != NULL && cb->is_nmethod()) ? (nmethod*)cb : NULL;
         if (nm != NULL && nm->has_unsafe_access()) {
           stub = StubRoutines::handler_for_unsafe_access();
         }