8329995: Restricted access to /proc can cause JFR initialization to…

… crash

Reviewed-by: shade
Backport-of: a92ad03946d296510c8c2ac18278608e8032b3f3

src/hotspot/os/linux/os_perf_linux.cpp

@@ -847,7 +847,7 @@ SystemProcessInterface::SystemProcesses::ProcessIterator::ProcessIterator() {
 bool SystemProcessInterface::SystemProcesses::ProcessIterator::initialize() {
   _dir = os::opendir("/proc");
   _entry = NULL;
-  _valid = true;
+  _valid = _dir != NULL; // May be null if /proc is not accessible.
   next_process();
 
   return true;