readelf: Workaround stringop-truncation error

readelf: Workaround stringop-truncation error

In function ‘strncpy’,
    inlined from ‘print_ehdr’ at readelf.c:1175:4:
error: ‘__builtin_strncpy’ specified bound 512 equals destination size
       [-Werror=stringop-truncation]

strncpy doesn't terminate the copied string if there is not enough
room. We compensate later by explicitly adding a zero terminator at
buf[sizeof (buf) - 1]. Normally gcc does see this, but with
-fsanitize=address there is too much (checking) code in between. But
it is actually better to not let strncpy do too much work, so
substract one from the size.

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