Fix the following warning/error with clang:

libexec/rtld-elf/rtld.c:1898:22: error: comparison between pointer and integer ('Elf_Addr *' (aka 'unsigned int *') and 'Elf_Addr' (aka 'unsigned int')) [-Werror]

if (preinit_addr == (Elf_Addr)NULL)
    ~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~

libexec/rtld-elf/rtld.c:2039:16: error: comparison between pointer and integer ('Elf_Addr *' (aka 'unsigned int *') and 'Elf_Addr' (aka 'unsigned int')) [-Werror]

if (init_addr != (Elf_Addr)NULL) {
    ~~~~~~~~~ ^  ~~~~~~~~~~~~~~

Reviewed by: kib