Fix use of uninitialized variables

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>

Public issue https://github.com/mackron/miniaudio/issues/355

Signed-off-by: Steven Noonan <steven@uplinklabs.net>

Signed-off-by: Steven Noonan <steven@uplinklabs.net>

Public issue https://github.com/mackron/miniaudio/issues/348

Public issue https://github.com/mackron/miniaudio/issues/347

Public issue https://github.com/mackron/miniaudio/issues/341

  * If starting the backend fails, don't attempt to enter the data loop
  * If stopping the backend fails, don't fire the stop callback.

Public issue https://github.com/mackron/miniaudio/issues/344

Public issue https://github.com/mackron/miniaudio/issues/341

This was happening because ma_log_level_to_string() was only being
included when MA_DEBUG_OUTPUT is enabled.

Fix `ma_log_postv` not returning anything under certain circumstances

The `__STDC_VERSION__ >= 199901L` and the `_MSC_VER >= 1200` code
had different styles for handling errors, so I've made them match.

MSVC 6 was printing a warning about this.

Public issue https://github.com/mackron/miniaudio/issues/340

Fix invalid vsnprintf buffer size