The ConvertAddr function doesn't reliably set the 'addr' return value,
and so callers are getting flagged for using potentially uninitialized
values. Initialize the value in the callers to NULL and then go ahead
and check for NULL values before using them.
Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>

v2: backport to nx-libs 3.6.x (Ulrich Sibiller)