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

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

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;
         }

test/t/exception.rb

@@ -400,7 +400,7 @@ assert('GC in rescue') do
     end
   rescue => exception
     GC.start
-    assert_equal("#{__FILE__}:#{line}:in call",
+    assert_equal("#{__FILE__}:#{line}",
                  exception.backtrace.first)
   end
 end
@@ -418,7 +418,7 @@ assert('Method call in rescue') do
   rescue => exception
     [3].each do
     end
-    assert_equal("#{__FILE__}:#{line}:in call",
+    assert_equal("#{__FILE__}:#{line}",
                  exception.backtrace.first)
   end
 end