Fixed backtrace message for top-level blocks; fix #4936

In top-level, `mid` is `NULL`. We used to ignore 'mid` update for `NULL`.

1 files changed, 1 insertions, 2 deletions

diff --git a/src/vm.c b/src/vm.c
index 2ccc03e30..09b0c767e 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -1500,8 +1500,7 @@ RETRY_TRY_BLOCK:
         mrb_sym mid;
         struct REnv *e = MRB_PROC_ENV(m);
 
-        mid = e->mid;
-        if (mid) ci->mid = mid;
+        ci->mid = mid = e->mid;
         if (!e->stack) {
           e->stack = mrb->c->stack;
         }