Don't repeat file that can't advance its position.

This avoids a confusing condition when hackrf_transfer is reading from
stdin with -R in which it does not repeat previous data but repeatedly
restarts reading new data after the user types Ctrl-D to indicate EOF.

The -R option should repeat previous data or should do nothing if the
input file cannot be rewound.

host/hackrf-tools/src/hackrf_transfer.c

@@ -543,7 +543,7 @@ int tx_callback(hackrf_transfer* transfer)
 	}
 
 	/* Otherwise, the file ran short. If not repeating, this is the last data. */
-	if (!repeat) {
+	if ((!repeat) || (ftell(file) < 1)) {
 		tx_complete = true;
 		return 0;
 	}