nir: fix warning in nir_lower_io.c

nir: fix warning in nir_lower_io.c

Initialize the variable with NULL.  Fixes the following

    In file included from ../src/compiler/nir/nir_lower_io.c:34:
    ../src/compiler/nir/nir_lower_io.c: In function ‘nir_lower_explicit_io’:
    ../src/compiler/nir/nir.h:668:11: warning: ‘addr’ may be used uninitialized in this function [-Wmaybe-uninitialized]
        return src;
               ^~~
    ../src/compiler/nir/nir_lower_io.c:735:17: note: ‘addr’ was declared here
        nir_ssa_def *addr;
                     ^~~~

v2: Avoid using a 'default' case so we get help from the compiler when
    new deref types are added. (Lionel)

Acked-by: Jason Ekstrand <jason@jlekstrand.net>