sdlaudio: always clear the sample buffer

Always fill the remaining audio callback buffer with silence.
SDL 2.0 doesn't initialize the audio callback buffer. This was
an incompatible change compared to SDL 1.2. For reference read
the SDL 1.2 to 2.0 migration guide.

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 9315afe5-5958-c0b4-ea1e-14769511a9d5@t-online.de
Message-Id: <20210110100239.27588-5-vr_qemu@t-online.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

audio/sdlaudio.c

@@ -211,27 +211,26 @@ static void sdl_callback (void *opaque, Uint8 *buf, int len)
     SDLAudioState *s = &glob_sdl;
     HWVoiceOut *hw = &sdl->hw;
 
-    if (s->exit) {
-        return;
-    }
+    if (!s->exit) {
 
-    /* dolog("callback: len=%d avail=%zu\n", len, hw->pending_emul); */
+        /* dolog("callback: len=%d avail=%zu\n", len, hw->pending_emul); */
 
-    while (hw->pending_emul && len) {
-        size_t write_len;
-        ssize_t start = ((ssize_t) hw->pos_emul) - hw->pending_emul;
-        if (start < 0) {
-            start += hw->size_emul;
-        }
-        assert(start >= 0 && start < hw->size_emul);
+        while (hw->pending_emul && len) {
+            size_t write_len;
+            ssize_t start = (ssize_t)hw->pos_emul - hw->pending_emul;
+            if (start < 0) {
+                start += hw->size_emul;
+            }
+            assert(start >= 0 && start < hw->size_emul);
 
-        write_len = MIN(MIN(hw->pending_emul, len),
-                        hw->size_emul - start);
+            write_len = MIN(MIN(hw->pending_emul, len),
+                            hw->size_emul - start);
 
-        memcpy(buf, hw->buf_emul + start, write_len);
-        hw->pending_emul -= write_len;
-        len -= write_len;
-        buf += write_len;
+            memcpy(buf, hw->buf_emul + start, write_len);
+            hw->pending_emul -= write_len;
+            len -= write_len;
+            buf += write_len;
+        }
     }
 
     /* clear remaining buffer that we couldn't fill with data */