8239069: C2: SIGSEGV in IdealGraphPrinter::walk_nodes due to C->root(…

…) being NULL

Fix NULL pointer dereference when trying to print the ideal graph when Compile::record_failure() is called twice.

Reviewed-by: neliasso, thartmann

src/hotspot/share/opto/idealGraphPrinter.cpp

@@ -664,7 +664,7 @@ void IdealGraphPrinter::print_method(const char *name, int level) {
 // Print current ideal graph
 void IdealGraphPrinter::print(const char *name, Node *node) {
 
-  if (!_current_method || !_should_send_method) return;
+  if (!_current_method || !_should_send_method || node == NULL) return;
 
   // Warning, unsafe cast?
   _chaitin = (PhaseChaitin *)C->regalloc();