vm: Initial support for calling bytecode functions w/o C stack ("stackless").
diff --git a/py/bc.c b/py/bc.c
index 3a8dc5e..af855e9 100644
--- a/py/bc.c
+++ b/py/bc.c
@@ -86,6 +86,9 @@
     mp_obj_fun_bc_t *self = self_in;
     mp_uint_t n_state = code_state->n_state;
 
+    #if MICROPY_STACKLESS
+    code_state->prev = NULL;
+    #endif
     code_state->code_info = self->bytecode;
     code_state->sp = &code_state->state[0] - 1;
     code_state->exc_sp = (mp_exc_stack_t*)(code_state->state + n_state) - 1;