api: null terminate readlink buffer in cgroup_get_procname_from_procfs()

api: null terminate readlink buffer in cgroup_get_procname_from_procfs()

Fix readlink buffer null termination warning, reported by Coverity tool:

CID 258273 (#2 of 2): String not null terminated (STRING_NULL).
string_null: Passing unterminated string buf to strdup, which expects a
null-terminated string.

As per the man pages (man 2 readlink):
"readlink() does not append a null byte to buf.  It  will (silently)
truncate the contents (to a length of bufsiz characters), in case the
buffer is too small to hold all of the contents."

Explicitly null terminate the buffer passed to readlink() in
cgroup_get_procname_from_procfs()

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>