rtlsdr_set_sample_rate(): don't access dev structure before checking for NULL

Thanks to Derek Lieber for reporting this bug.

Signed-off-by: Steve Markgraf <steve@steve-m.de>
lib: reset device if needed

This improves the situation on hosts with flaky host
controllers like the Raspberry Pi and the WR703N.

Signed-off-by: Steve Markgraf <steve@steve-m.de>
rtl_adsb: improve sensitivity, stability

Signed-off-by: Steve Markgraf <steve@steve-m.de>
rtl_adsb: 16 bit magnitudes