libelf: Only fetch shdr once in elf_compress[_gnu]

libelf: Only fetch shdr once in elf_compress[_gnu]

Some compilers assume the second call to elf[32|64]_getshdr can fail
and produce error: potential null pointer dereference. Just store the
result of the first call and reuse (when not NULL).

       * libelf/elf_compress.c (elf_compress): Store getshdr result in
       a shdr union var.
       * libelf/elf_compress_gnu.c (): Likewise

https://sourceware.org/bugzilla/show_bug.cgi?id=32311

Signed-off-by: Mark Wielaard <mark@klomp.org>