Modify jl_nth_union_component to be more robust in the case of trying…

… to get an invalid 'n'. This can happen if a struct or array containing Unions gets allocated using 'uninitialized' memory and the corresponding union selector bytes may contain arbitrary values. In these cases, we always know that u->a (where 'u' is a Union type) is a valid, non-Union type, so just return that instead. Fixes #25310

src/jltypes.c

@@ -367,7 +367,12 @@ static jl_value_t *nth_union_component(jl_value_t *v, int *pi)
 
 jl_value_t *jl_nth_union_component(jl_value_t *v, int i)
 {
-    return nth_union_component(v, &i);
+    jl_value_t *ret = nth_union_component(v, &i);
+    if (!ret) {
+        jl_uniontype_t *u = (jl_uniontype_t *)v;
+        return u->a;
+    }
+    return ret;
 }
 
 // inverse of jl_nth_union_component