perf buildid: Fix possible unterminated readlink() result buffer

perf buildid: Fix possible unterminated readlink() result buffer

The readlink function doesn't guarantee that a '\0' will be put at the
end of the provided buffer if there is no space left.

No need to do "buf[len] = '\0';" since the buffer is allocated with
zalloc().

Link: http://lkml.kernel.org/r/4E986ABF.9040706@intra2net.com
Signed-off-by: Thomas Jarosch <thomas.jarosch@intra2net.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>