nth_methtable：variable n may be 0, we will get the index value of tuple as -1

[awfeequdng]

in latest master method.c, in function nth_methtable, variable n may be 0, we will get the index value of tuple as -1

[vtjnash]

Looking closer, I realized that jl_is_tuple_type(a) implies a->name == jl_tuple_typename and jl_tuple_typename->mt != NULL, so this was not truly ever able to be reached. But better to be clear about that intent in the code (#47666).