Fix another infinite loop in mix_source_buffer(), case of uneven input

This fixes another potential infinite loop, which may occur if remaining data in the input is less than `bufferframesize`, in such case `framesavail` resulted in 0, and src->offset never advanced.
There are seemingly two options here:
1. Skip the remainder.
2. Use a local temp buffer, copy the remainder over and fill necessary remaining bytes with zeroes, then pass it into mix_buffer() too.

I chose the first, as this ever affects only 1 last sample which, apparently, does not have all of the channels data filled, so this does not seem to be worth it.

Author: ivan-mogilko

mojoal.c

@@ -1471,6 +1471,11 @@ static ALboolean mix_source_buffer(ALCcontext *ctx, ALsource *src, BufferQueueIt
             const int mixframes = SDL_min(framesneeded, framesavail);
             mix_buffer(src, buffer, src->panning, data, *stream, mixframes);
             src->offset += mixframes * bufferframesize;
+            /* workaround in case remains are less than bufferframesize:
+            in this case it's easier to just skip the remaining sample */
+            if ((src->offset < buffer->len) && ((buffer->len - src->offset) < bufferframesize)) {
+                src->offset = buffer->len;
+            }
             *len -= mixframes * deviceframesize;
             *stream += mixframes * ctx->device->channels;
         }