Fix VM event error handling for finalizers.

Reported by Sergey Kaplun.

(cherry picked from commit fbb36bb)

After the previous commit, the VM handler for the 'errfin' VM event
works incorrectly. The error object is taken not from the stack on which
the VM handler is invoked. Hence, it breaks the non-VM stack and returns
an incorrect error message.

This patch fixes the issue by copying the object from the corresponding
Lua stack.

Sergey Kaplun:
* added the description and the test for the problem

Part of tarantool/tarantool#12134

Author: Mike Pall

src/lj_gc.c

@@ -519,10 +519,12 @@ static void gc_call_finalizer(global_State *g, lua_State *L,
   if (LJ_HASPROFILE && (oldh & HOOK_PROFILE)) lj_dispatch_update(g);
   g->gc.threshold = oldt;  /* Restore GC threshold. */
   if (errcode) {
+    TValue tmp;
+    copyTV(VL, &tmp, VL->top-1);
+    VL->top--;
     lj_vmevent_send(g, ERRFIN,
-      copyTV(V, V->top++, L->top-1);
+      copyTV(V, V->top++, &tmp);
     );
-    L->top--;
   }
 }
 

test/tarantool-tests/lj-1445-errfin-errmsg.test.lua

@@ -0,0 +1,27 @@
+local tap = require('tap')
+local test = tap.test('lj-1445-errfin-errmsg')
+
+test:plan(2)
+
+local EXPECTED_ERR = 'expected err'
+local function bad_fin()
+  error(EXPECTED_ERR)
+end
+local EXPECTED_LOCATION = debug.getinfo(bad_fin).linedefined + 1
+
+local match_err = false
+local match_line = false
+local function errfin_handler(errmsg)
+  match_err = errmsg:match(EXPECTED_ERR)
+  match_line = errmsg:match(EXPECTED_LOCATION)
+end
+
+jit.attach(errfin_handler, 'errfin')
+
+debug.getmetatable(newproxy(true)).__gc = bad_fin
+collectgarbage()
+
+test:ok(match_err, 'correct error message in errfin handler')
+test:ok(match_line, 'correct source line in errfin handler')
+
+test:done(true)