Silence compiler warning

Silence compiler warning

Static analysis claims that ptr += size may assign garbage. But since the
protocol requires format to be 8, 16 or 32, size should always have a valid
value. Initialize to 0 to shut up clang.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Jeremy Huddleston <jeremyhu@apple.com>