Make sure the frame usecapture points at is ref'd.

We kinda got away with not doing this before, when the cur_usecapture
could never be collected. Now they can be when a thread terminates, so
protection is essential.

src/6model/reprs/MVMCallCapture.c

@@ -71,6 +71,10 @@ static void gc_free(MVMThreadContext *tc, MVMObject *obj) {
             MVM_checked_free_null(ctx->body.apc);
         }
     }
+    else {
+        if (ctx->body.use_mode_frame)
+            MVM_frame_dec_ref(tc, ctx->body.use_mode_frame);
+    }
 }
 
 /* Gets the storage specification for this representation. */

src/6model/reprs/MVMCallCapture.h

@@ -9,6 +9,10 @@ struct MVMCallCaptureBody {
      * the frame in question. For save mode, we allocate a fresh one. */
     MVMArgProcContext *apc;
 
+    /* The frame the ArgProcContext lives in, if we're in use mode. This
+     * ensures the frame stays alive long enough. */
+    MVMFrame *use_mode_frame;
+
     /* The effective MVMCallsite. This may be the original one, but in the
      * event of flattening will describe the flattened outcome. */
     MVMCallsite *effective_callsite;

src/core/args.c

@@ -79,8 +79,11 @@ MVMCallsite * MVM_args_proc_to_callsite(MVMThreadContext *tc, MVMArgProcContext
 /* Puts the args passed to the specified frame into the current use_capture. */
 MVMObject * MVM_args_use_capture(MVMThreadContext *tc, MVMFrame *f) {
     MVMCallCapture *capture = (MVMCallCapture *)tc->cur_usecapture;
-    capture->body.mode = MVM_CALL_CAPTURE_MODE_USE;
-    capture->body.apc  = &f->params;
+    if (capture->body.use_mode_frame)
+        MVM_frame_dec_ref(tc, capture->body.use_mode_frame);
+    capture->body.mode               = MVM_CALL_CAPTURE_MODE_USE;
+    capture->body.use_mode_frame     = MVM_frame_inc_ref(tc, f);
+    capture->body.apc                = &f->params;
     capture->body.effective_callsite = MVM_args_proc_to_callsite(tc, &f->params);
     return tc->cur_usecapture;
 }