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.

(And also, we should not enter an infinite loop, caused by applying
an patch I sent that didn't also change the while condition.)

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