lua: use cur_L in cord_on_yield

Before the patch, fiber->storage.lua.stack is used for
`panic` calls. However, some fibers don't have any Lua
state saved in their storage (for example, space
triggers).

After the patch, the Lua state pointed by `cur_L` is
used to make those calls, as it is always present.

Closes #6647
NO_DOC=bugfix