Fix some VC6/2003 errors

For whatever reason, neither compiler likes implicit casts between
`volatile unsigned int*` and `int*`.

miniaudio.h

@@ -10409,7 +10409,7 @@ static ma_thread_result MA_THREADCALL ma_device_thread__null(void* pData)
             }
 
             /* Getting here means a suspend or kill operation has been requested. */
-            c89atomic_exchange_32(&pDevice->null_device.operationResult, MA_SUCCESS);
+            c89atomic_exchange_32((c89atomic_uint32*)&pDevice->null_device.operationResult, MA_SUCCESS);
             ma_event_signal(&pDevice->null_device.operationCompletionEvent);
             continue;
         }
@@ -10423,7 +10423,7 @@ static ma_thread_result MA_THREADCALL ma_device_thread__null(void* pData)
             ma_timer_init(&pDevice->null_device.timer);
 
             /* We're done. */
-            c89atomic_exchange_32(&pDevice->null_device.operationResult, MA_SUCCESS);
+            c89atomic_exchange_32((c89atomic_uint32*)&pDevice->null_device.operationResult, MA_SUCCESS);
             ma_event_signal(&pDevice->null_device.operationCompletionEvent);
             continue;
         }
@@ -10431,7 +10431,7 @@ static ma_thread_result MA_THREADCALL ma_device_thread__null(void* pData)
         /* Killing the device means we need to get out of this loop so that this thread can terminate. */
         if (pDevice->null_device.operation == MA_DEVICE_OP_KILL__NULL) {
             c89atomic_exchange_32(&pDevice->null_device.operation, MA_DEVICE_OP_NONE__NULL);
-            c89atomic_exchange_32(&pDevice->null_device.operationResult, MA_SUCCESS);
+            c89atomic_exchange_32((c89atomic_uint32*)&pDevice->null_device.operationResult, MA_SUCCESS);
             ma_event_signal(&pDevice->null_device.operationCompletionEvent);
             break;
         }
@@ -10439,7 +10439,7 @@ static ma_thread_result MA_THREADCALL ma_device_thread__null(void* pData)
         /* Getting a signal on a "none" operation probably means an error. Return invalid operation. */
         if (pDevice->null_device.operation == MA_DEVICE_OP_NONE__NULL) {
             MA_ASSERT(MA_FALSE);  /* <-- Trigger this in debug mode to ensure developers are aware they're doing something wrong (or there's a bug in a miniaudio). */
-            c89atomic_exchange_32(&pDevice->null_device.operationResult, (c89atomic_uint32)MA_INVALID_OPERATION);
+            c89atomic_exchange_32((c89atomic_uint32*)&pDevice->null_device.operationResult, (c89atomic_uint32)MA_INVALID_OPERATION);
             ma_event_signal(&pDevice->null_device.operationCompletionEvent);
             continue;   /* Continue the loop. Don't terminate. */
         }