LUAU_ASSERT failure in luaG_getline if called from lua_Alloc

[shaggie76]

We have a profile mode where we pass a custom allocator that uses lua_getinfo(L, frame, "snl", &in) to get callstacks for where we're generating the most garbage.

Under normal operation this works great however if you enable lua_singlestep and start getting luau_callhook calls it can sometimes crash.

The problem occurs when luaD_checkstack needs to reallocate before L->ci->savedpc has been incremented: the allocator tries to call lua_getinfo which then runs into the LUAU_ASSERT in luaG_getline because pc == -1.

We fixed luau_callhook locally by simply moving the luaD_checkstack down below where savedpc is updated, like this:

    // note: the pc expectations of the hook are matching the general "pc points to next instruction"
    // however, for the hook to be able to continue execution from the same point, this is called with savedpc at the *current* instruction
    if (L->ci->savedpc)
        L->ci->savedpc++;

    luaD_checkstack(L, LUA_MINSTACK); // ensure minimum stack size
    L->ci->top = L->top + LUA_MINSTACK;
    LUAU_ASSERT(L->ci->top <= L->stack_last);

It's a simple fix and hopefully it helps someone else doing similar tricks.