rtspconnection: Handle invalid argument properly
In case when conn->input_stream is NULL and glib was built with "glib_checks" enabled, g_pollable_input_stream_read_nonblocking() returns -1, but does not set the "err".
The call stack: read_bytes() -> fill_bytes() -> fill_raw_bytes()
The return value -1 passed up to read_bytes() and incorrectly processed there after "error:" label.
This changes the return value to EINVAL.