Allow abort_to_prompt to avoid a longjmp

* libguile/vm-engine.c (abort):
* libguile/vm.c (abort_to_prompt): Allow fallthrough if longjmp isn't
  needed.

libguile/vm-engine.c

@@ -788,8 +788,12 @@ VM_NAME (scm_thread *thread, jmp_buf *registers, int resume)
       SYNC_IP ();
       scm_vm_intrinsics.abort_to_prompt (thread, registers);
 
-      /* vm_abort should not return */
-      abort ();
+      /* If abort_to_prompt returned, that means there were no
+         intervening C frames to jump over, so we just continue
+         directly.  */
+      CACHE_REGISTER ();
+      ABORT_CONTINUATION_HOOK ();
+      NEXT (0);
     }
 
   /* builtin-ref dst:12 idx:12

libguile/vm.c

@@ -1403,11 +1403,11 @@ abort_to_prompt (scm_thread *thread, jmp_buf *current_registers)
   vp->sp = sp;
   vp->ip = ip;
 
-  /* Jump! */
+  /* If there are intervening C frames, then jump over them, making a
+     nonlocal exit.  Otherwise fall through and let the VM pick up where
+     it left off.  */
+  if (current_registers != registers)
     longjmp (*registers, 1);
-
-  /* Shouldn't get here */
-  abort ();
 }
 
 SCM