8275049: [ZGC] missing null check in ZNMethod::log_register

Backport-of: cf828673a9b76fd3f26db9a3f714166861a65c9e

Author: TheRealMDoerr

src/hotspot/share/gc/z/zNMethod.cpp

@@ -126,8 +126,10 @@ void ZNMethod::log_register(const nmethod* nm) {
     oop* const begin = nm->oops_begin();
     oop* const end = nm->oops_end();
     for (oop* p = begin; p < end; p++) {
+      const oop o = Atomic::load(p); // C1 PatchingStub may replace it concurrently.
+      const char* external_name = (o == nullptr) ? "N/A" : o->klass()->external_name();
       log_oops.print("           Oop[" SIZE_FORMAT "] " PTR_FORMAT " (%s)",
-                     (p - begin), p2i(*p), (*p)->klass()->external_name());
+                     (p - begin), p2i(o), external_name);
     }
   }