tests/sw_tree1.c: fix unitialized saveptr

[blmaier]

Building the dtc tests on the Conda build system results in the following error.

In function '__strtok_r_1c', 2024-11-23T19:17:20.7930512Z inlined from 'main' at ../tests/sw_tree1.c:140:17:
$BUILD_PREFIX/x86_64-conda-linux-gnu/sysroot/usr/include/bits/string2.h:1177:10: error: 'saveptr' may be used uninitialized [-Werror=maybe-uninitialized]
1177 | while (*__s == __sep)
| ^~~~
../tests/sw_tree1.c: In function 'main':
../tests/sw_tree1.c:137:39: note: 'saveptr' was declared here
137 | char *str = argv[2], *saveptr, *tok;
| ^~~~~~~
cc1: all warnings being treated as errors

The manpage strtok(3) says the following.

VERSIONS
On some implementations, *saveptr is required to be NULL on the first call to strtok_r() that is being used to parse str.

So set it to NULL.