x86 kernel stack trace: break when return address is zero

* No need to continue walking the stack when a zero return
  address is found, only bogus frames will follow

* Fixes KDLs seen when using the system profiler, due to
  a bug that will be fixed in the next commit...

src/system/kernel/arch/x86/arch_debug.cpp

@@ -1167,6 +1167,9 @@ arch_debug_get_stack_trace(addr_t* returnAddresses, int32 maxCount,
 				break;
 			}
 		}
+		
+		if (ip == 0)
+			break;
 
 		if (skipFrames <= 0
 			&& ((flags & STACK_TRACE_KERNEL) != 0 || onKernelStack)) {