Fix memory corruption in S_TransferPaintBuffer

When using a non-default sound configuration (such as 6 channels), after
a long time (about 4.5hours for 6 channels at 22050 Hz) an overflow will
occur in `S_TransferPaintBuffer`, causing an out of bounds write into
the dma buffer.

The problematic line is:
```
out_idx = (s_paintedtime * dma.channels) % dma.samples;
```

With `s_paintedtime` large enough, the result of the multiplication will
overflow to a negative number (since `s_paintedtime` is signed), and the
index into the output buffer will be negative.

code/client/snd_mix.c

@@ -176,7 +176,7 @@ void S_TransferPaintBuffer(int endtime, portable_samplepair_t paintbuffer[])
 	{	// general case
 		p = (int *) paintbuffer;
 		count = (endtime - s_paintedtime) * dma.channels;
-		out_idx = (s_paintedtime * dma.channels) % dma.samples;
+		out_idx = ((unsigned int)s_paintedtime * dma.channels) % dma.samples;
 		step = 3 - MIN(dma.channels, 2);
 
 		if ((dma.isfloat) && (dma.samplebits == 32))