Panic on attempt to index a string value

Please answer the following before submitting your issue:

• What version of GopherLua are you using? : ccacf662c9d24821ea6cb1abaa515cc3b6aad7c6
• What version of Go are you using? : 1.23.6
• What operating system and processor architecture are you using? : Ubuntu 24.04 amd64
• What did you do? : See below
• What did you expect to see? : See below
• What did you see instead? : See below

The Lua source code below

debug.setmetatable('string', {})
do ('Lua').key = 1 end;

triggers a panic in GopherLua:

$ /home/sergeyb/sources/go/bin/glua -e "
debug.setmetatable('string', {})
do ('Lua').key = 1 end;
"
runtime error: invalid memory address or nil pointer dereference
stack traceback:
        <string>:3: in main chunk
        [G]: ?

The same snippet executed by Lua 5.1 reports an error "attempt to index a string value":

$ lua5.1 -e "
debug.setmetatable('string', {})
do ('Lua').key = 1 end;
"
lua5.1: (command line):3: attempt to index a string value
stack traceback:
        (command line):3: in main chunk
        [C]: ?