rtl_fm: fix scanning

Signed-off-by: Steve Markgraf <steve@steve-m.de>
rtl_adsb: Fix invalid memory access

single_manchester() considers both i and i+1, but the loop only
tests that i is in bounds. This causes undefined behavior, including
but not limited to a SIGBUS-related crash on Mac OS X.

Signed-off-by: Steve Markgraf <steve@steve-m.de>