Guard ruby_current_vm_ptr is non-null in sigaction

ext/stackprof/stackprof.c

@@ -25,6 +25,8 @@
 #define FAKE_FRAME_MARK  INT2FIX(1)
 #define FAKE_FRAME_SWEEP INT2FIX(2)
 
+extern void* ruby_current_vm_ptr;
+
 static const char *fake_frame_cstrs[] = {
 	"(garbage collection)",
 	"(marking)",
@@ -725,6 +727,11 @@ stackprof_signal_handler(int sig, siginfo_t *sinfo, void *ucontext)
 
     if (!_stackprof.running) return;
 
+    // There's a possibility that the signal handler is invoked *after* the Ruby
+    // VM has been shut down (e.g. after ruby_cleanup(0)). In this case, things
+    // that rely on global VM state (e.g. rb_during_gc) will segfault.
+    if (ruby_current_vm_ptr == NULL) return;
+
     if (_stackprof.mode == sym_wall) {
         // In "wall" mode, the SIGALRM signal will arrive at an arbitrary thread.
         // In order to provide more useful results, especially under threaded web