Fix memory leak on OOM in virSecuritySELinuxMCSFind

The body of the loop in virSecuritySELinuxMCSFind would
directly 'return NULL' on OOM, instead of jumping to the
cleanup label. This caused a leak of several local vars.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
(cherry picked from commit f2d8190c)

src/security/security_selinux.c

@@ -214,7 +214,7 @@ virSecuritySELinuxMCSFind(virSecurityManagerPtr mgr)
         if (c1 == c2) {
             if (virAsprintf(&mcs, "%s:c%d", sens, catMin + c1) < 0) {
                 virReportOOMError();
-                return NULL;
+                goto cleanup;
             }
         } else {
             if (c1 > c2) {
@@ -224,7 +224,7 @@ virSecuritySELinuxMCSFind(virSecurityManagerPtr mgr)
             }
             if (virAsprintf(&mcs, "%s:c%d,c%d", sens, catMin + c1, catMin + c2) < 0) {
                 virReportOOMError();
-                return NULL;
+                goto cleanup;
             }
         }