util: Fix pointer to integer conversion error when using libunwind

../src/util/u_debug_stack.c:97:20: error: incompatible pointer to integer conversion assigning to 'unw_word_t' (aka 'unsigned long') from 'void *' [-Werror,-Wint-conversion]
   pip.unwind_info = NULL;
                   ^ ~~~~
1 error generated.

Fixes: 70c272004f72 "gallium/util: libunwind support"
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/4094

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8507>
(cherry picked from commit 08ac7b52e7814540e92d5add29049324f6841bb5)
2 files changed