Fix potential missing gc root in check_disabled_ambiguous_visitor
```
/home/keno/julia-old/src/gf.c:1220:30: note: Started tracking value here
jl_value_t *isect2 = jl_type_intersection((jl_value_t*)mth->sig, (jl_value_t*)sig);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/keno/julia-old/src/gf.c:1223:13: note: Passing non-rooted value as argument to function that may GC
if (jl_types_equal(isect, isect2)) {
^ ~~~~~~
```
