Warnings about uninitialized variables in `bessel_ik`

[Lastique]

Building Boost.Math 1.86.0 on Ubuntu 24.04, gcc 13.2 produces these warnings:

In file included from ./boost/math/special_functions/bessel.hpp:24,
                 from ./boost/math/special_functions/airy.hpp:12,
                 from ./boost/math/special_functions.hpp:15,
                 from libs/math/build/../src/tr1/pch.hpp:9:
./boost/math/special_functions/detail/bessel_ik.hpp: In function ‘boost_cyl_bessel_k’:
./boost/math/special_functions/detail/bessel_ik.hpp:417:11: warning: ‘u’ may be used uninitialized [-Wmaybe-uninitialized]
  417 |         T z = (u + n % 2);
      |           ^
./boost/math/special_functions/detail/bessel_ik.hpp:304:7: note: ‘u’ was declared here
  304 |     T u, Iv, Kv, Kv1, Ku, Ku1, fv;
      |       ^
./boost/math/special_functions/detail/bessel_ik.hpp:417:22: warning: ‘n’ may be used uninitialized [-Wmaybe-uninitialized]
  417 |         T z = (u + n % 2);
      |                    ~~^~~
./boost/math/special_functions/detail/bessel_ik.hpp:307:14: note: ‘n’ was declared here
  307 |     unsigned n, k;
      |              ^

From a brief look these look legitimate, but I didn't investigate deeply.

[mborland]

Should be fixed by: #1111. Further discussion is in: #1181