llc: remove NULL-pointer check of gprs_llc_queue::size()/octets()

All callers now check the pointer before calling it.
gcc6 is optimizing `if (!this) {CODE}` as this is assumed to never be a
std::nullptr here.

Change-Id: I918a094e0dc59098a9eb00d152c9ae42d36b3a99

src/llc.h

@@ -127,10 +127,10 @@ inline bool gprs_llc::fits_in_current_frame(uint8_t chunk_size) const
 
 inline size_t gprs_llc_queue::size() const
 {
-	return this ? m_queue_size : 0;
+	return m_queue_size;
 }
 
 inline size_t gprs_llc_queue::octets() const
 {
-	return this ? m_queue_octets : 0;
+	return m_queue_octets;
 }