Fixed compiling for iOS 11.x and earlier

Public issue https://github.com/mackron/miniaudio/issues/363

It looks like IOID will be null when both modes are supported.

Public issue https://github.com/mackron/miniaudio/issues/365

Fix use of uninitialized variables

Fixed compilation warnings & errors on iOS

This applies for resource manager data source and sounds.

This addresses an issue where data streams can have a delay between
loop points due to it's asynchronous loading mechanism. With this
change, streams should now have a clean loop transition.

When shutting down, the resultALSA variable can be used when
uninitialized in the ma_device_read__alsa and ma_device_write__alsa
routines.  This has been verified by both gcc warnings and in
application code (we were occasionally seeing segfaults when our
callback routine was being passed a crazy large value for the number of
frames read when shutting down).
Signed-off-by: Charles Steinkuehler <charles@steinkuehler.net>

This is used for giving the data source a hint as to whether or not
it'll be used in a looping situations. It's required for some data
sources which need to know about it's looping state for caching or
whatnot.

The API for the resource management part of this commit may change.