Actually use number correctly casted from double to int as index

The code was using (int)jv_number_value(k) instead of didx.

follow-up from 0e067ef

Useless assignments to didx detected by clang-tidy.

src/jv_aux.c

@@ -95,10 +95,9 @@ jv jv_get(jv t, jv k) {
       } else {
         if (didx < INT_MIN) didx = INT_MIN;
         if (didx > INT_MAX) didx = INT_MAX;
-        int idx = (int)jv_number_value(k);
-        if (idx < 0)
-          idx += jv_array_length(jv_copy(t));
-        v = jv_array_get(t, idx);
+        if (didx < 0)
+          didx += jv_array_length(jv_copy(t));
+        v = jv_array_get(t, didx);
         if (!jv_is_valid(v)) {
           jv_free(v);
           v = jv_null();