mruby-bigint: fix carry placement in uadd()

The final carry was stored at z->p[y->sz], but when x is larger
than y, this index falls within the already-computed result and
corrupts it. Store at z->p[i] instead, which correctly points to
max(x->sz, y->sz) after all loops complete.

This bug caused incorrect results when adding a small number to
an all-ones number with 1124+ limbs (35968+ bits).

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-bigint/core/bigint.c

@@ -432,8 +432,8 @@ uadd(mpz_t *z, mpz_t *x, mpz_t *y)
     c >>= DIG_SIZE;
   }
 
-  /* Store final carry */
-  z->p[y->sz] = (mp_limb)c;
+  /* Store final carry at correct position (after all limbs) */
+  z->p[i] = (mp_limb)c;
 }
 
 /* z = y - x, ignoring sign */