engine: fix memory leak in netchan

frag_message_buf wasn't always freed alongside with fragbuf_t
instead of ensuring frag_message_buf being freed, I turned it into
a flexible array member

engine/common/netchan.h

@@ -184,13 +184,13 @@ typedef struct fragbuf_s
 	struct fragbuf_s	*next;				// next buffer in chain
 	int		bufferid;				// id of this buffer
 	sizebuf_t		frag_message;			// message buffer where raw data is stored
-	byte		*frag_message_buf;	// the actual data sits here
 	qboolean		isfile;				// is this a file buffer?
 	qboolean		isbuffer;				// is this file buffer from memory ( custom decal, etc. ).
 	qboolean		iscompressed;			// is compressed file, we should using filename.ztmp
 	char		filename[MAX_OSPATH];		// name of the file to save out on remote host
 	int		foffset;				// offset in file from which to read data
 	int		size;				// size of data to read at that offset
+	byte frag_message_buf[1]; // the actual data sits here (flexible)
 } fragbuf_t;
 
 // Waiting list of fragbuf chains