cgroup.c: don't leak mem+FD upon OOM

* src/util/cgroup.c (virCgroupDetectPlacement): Close the mapping
FILE* also upon error.

src/util/cgroup.c

@@ -183,6 +183,7 @@ static int virCgroupDetectPlacement(virCgroupPtr group)
     return 0;
 
 no_memory:
+    fclose(mapping);
     return -ENOMEM;
 
 }