Fix test failure

The GError object has to initialized to NULL before being passed to
g_file_set_contents or it will throw the following critical warning

GLib-CRITICAL **: g_file_set_contents: assertion 'error == NULL || *error == NULL' failed

and return FALSE, which leads to failed assertion and subsequently
to the test failing
Minor whitespace fix.

Really just for testing some git hooks.
filter.c: Fix endianness bug in sr_filter_probes
filter.c: Work around const warning.
scpi_serial: Iterate serial_write as necessary to send full commands.
rigol-ds: close SCPI device after using for scan.
Route sr_source_add for all serial devices through a serial_source_add wrapper.
Route sr_source_remove for all serial devices through a wrapper.
Add std_serial_dev_close() function.
Use std_serial_dev_close() to replace matching dev_close functions.
Use sdi->status instead of serial->fd to tell if port needs closing.
brymen-dmm: Use port name instead of fd in debug message.
Use sp_get_port_handle to get an fd for adding/removing serial sources.
Eliminate internal usage of serial->fd in serial.c.
ols: Get fd from sp_get_port_handle() rather than serial struct.
Remove now-unused fd field from struct sr_serial_dev_inst.
Disable OLS driver on Windows.