mruby-bigint: fix missing null terminator in D&C to_s

mpz_to_s_dc_recur fills in exactly num_digits characters but did not
add a null terminator. This caused valgrind errors when strlen was
called on the resulting string.

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

mrbgems/mruby-bigint/core/bigint.c

@@ -2755,6 +2755,9 @@ mpz_to_s_dc(mpz_ctx_t *ctx, char *s, mpz_t *x)
     /* Do the recursive conversion */
     mpz_to_s_dc_recur(ctx, s, &tmp, num_digits, pow5, num_powers, &scratch);
 
+    /* Null-terminate the string */
+    s[num_digits] = '\0';
+
     mrb->jmp = prev_jmp;
   } MRB_CATCH(&c_jmp) {
     mrb->jmp = prev_jmp;