backend-native: Fix stack corruption reading RFCOMM AT+BIA= response

On-stack buffer size is too short for sscanf to read AT+BIA= and AT+BAC=
response strings which have variable length.

Fix this by replacing sscanf with proper copy of input string.

Fixes: cca0d693 ("bluetooth: add AT+BIA support")
Part-of: <pulseaudio/pulseaudio!755>
2 jobs for pr/backend-native-fix-hfp-rfcomm-stack in 1 minute and 55 seconds (queued for 1 second)