Various fixes detected using PVS-Studio static analyzer.

- Add FIXME tags comments to some unfixed potential bugs
- Remove some checks (always false: unsigned never < 0)
- Fix some if statements based on reviews.
- Bunch of missing `else` statements

core/io/stream_peer.cpp

@@ -492,6 +492,8 @@ Error StreamPeerBuffer::get_partial_data(uint8_t* p_buffer, int p_bytes,int &r_r
 
 	PoolVector<uint8_t>::Read r = data.read();
 	copymem(p_buffer,r.ptr(),r_received);
+	
+	// FIXME: return what? OK or ERR_*
 }
 
 int StreamPeerBuffer::get_available_bytes() const {