Test the current thread's frame in heap snapshot

Fixes a bug that can in the best case cause a SEGV (which is how I
discovered it), and in the worst case lead to missing data in the
report.

src/profiler/heapsnapshot.c

@@ -513,7 +513,8 @@ static void process_workitems(MVMThreadContext *tc, MVMHeapSnapshotState *ss) {
             case MVM_SNAPSHOT_COL_KIND_THREAD_ROOTS: {
                 MVMThreadContext *thread_tc = (MVMThreadContext *)item.target;
                 MVM_gc_root_add_tc_roots_to_worklist(thread_tc, NULL, ss);
-                if (thread_tc->cur_frame && !MVM_FRAME_IS_ON_CALLSTACK(thread_tc, tc->cur_frame))
+                if (thread_tc->cur_frame &&
+                        !MVM_FRAME_IS_ON_CALLSTACK(thread_tc, thread_tc->cur_frame))
                     add_reference_const_cstr(tc, ss, "Current frame",
                         get_frame_idx(tc, ss, thread_tc->cur_frame));
                  break;