daemon/cgrulesengd: check the bytes read in cgre_receive_unix_domain_msg()

daemon/cgrulesengd: check the bytes read in cgre_receive_unix_domain_msg()

Fix ignoring the number of bytes read, warning reported by Coverity
tool:

CID 258286 (#1 of 1): Ignoring number of bytes read (CHECKED_RETURN).
check_return: read(int, void *, size_t) returns the number of bytes
read, but it is ignored.

In cgre_receive_unix_domain_msg(), the number of bytes read() is
ignored, while reading from the flag value of the pid. Coverity warns on
not checking the number of bytes read, fix it.

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