cgroup: don't give not null terminated string into strtoll()

CID 73345 (#1 of 1): String not null terminated (STRING_NULL)
2. string_null_argument: Function fread does not terminate string *buf. [Note: The source code implementation of the function has been overridden by a builtin model.]

Cc: Tycho Andersen <tycho.andersen@canonical.com>

cgroup.c

@@ -316,7 +316,7 @@ static int read_cgroup_prop(struct cgroup_prop *property, const char *fullpath)
 	}
 
 	memset(buf, 0, sizeof(buf));
-	if (fread(buf, sizeof(buf), 1, f) != 1) {
+	if (fread(buf, sizeof(buf) - 1, 1, f) != 1) {
 		if (!feof(f)) {
 			pr_err("Failed scanning %s\n", fullpath);
 			fclose(f);