8345505: Fix -Wzero-as-null-pointer-constant warnings in zero code

Reviewed-by: dholmes

Author: Kim Barrett

src/hotspot/cpu/zero/frame_zero.cpp

@@ -125,10 +125,10 @@ bool frame::safe_for_sender(JavaThread *thread) {
 bool frame::is_interpreted_frame_valid(JavaThread *thread) const {
   assert(is_interpreted_frame(), "Not an interpreted frame");
   // These are reasonable sanity checks
-  if (fp() == 0 || (intptr_t(fp()) & (wordSize-1)) != 0) {
+  if (fp() == nullptr || (intptr_t(fp()) & (wordSize-1)) != 0) {
     return false;
   }
-  if (sp() == 0 || (intptr_t(sp()) & (wordSize-1)) != 0) {
+  if (sp() == nullptr || (intptr_t(sp()) & (wordSize-1)) != 0) {
     return false;
   }
   // These are hacks to keep us out of trouble.