Fix invalid read in virCgroupGetValueStr

Don't check for '\n' at the end of file if zero bytes were read.

Found by valgrind:
==404== Invalid read of size 1
==404==    at 0x529B09F: virCgroupGetValueStr (vircgroup.c:540)
==404==    by 0x529AF64: virCgroupMoveTask (vircgroup.c:1079)
==404==    by 0x1EB475: qemuSetupCgroupForEmulator (qemu_cgroup.c:1061)
==404==    by 0x1D9489: qemuProcessStart (qemu_process.c:3801)
==404==    by 0x18557E: qemuDomainObjStart (qemu_driver.c:5787)
==404==    by 0x190FA4: qemuDomainCreateWithFlags (qemu_driver.c:5839)

Introduced by 0d0b4098.

https://bugzilla.redhat.com/show_bug.cgi?id=978356
(cherry picked from commit 306c49ff)

src/util/vircgroup.c

@@ -537,7 +537,7 @@ static int virCgroupGetValueStr(virCgroupPtr group,
         VIR_DEBUG("Failed to read %s: %m\n", keypath);
     } else {
         /* Terminated with '\n' has sometimes harmful effects to the caller */
-        if ((*value)[rc - 1] == '\n')
+        if (rc > 0 && (*value)[rc - 1] == '\n')
             (*value)[rc - 1] = '\0';
 
         rc = 0;