alsa: limit buffer_size to 32768 frames.

In testing, the file output plugin gave a max buffer size of about 20 million
frames, which is way more than what is really needed and causes a memory
allocation error on my system.

libavdevice/alsa-audio-common.c

@@ -260,6 +260,7 @@ av_cold int ff_alsa_open(AVFormatContext *ctx, snd_pcm_stream_t mode,
     }
 
     snd_pcm_hw_params_get_buffer_size_max(hw_params, &buffer_size);
+    buffer_size = FFMIN(buffer_size, ALSA_BUFFER_SIZE_MAX);
     /* TODO: maybe use ctx->max_picture_buffer somehow */
     res = snd_pcm_hw_params_set_buffer_size_near(h, hw_params, &buffer_size);
     if (res < 0) {

libavdevice/alsa-audio.h

@@ -40,6 +40,8 @@
         other formats */
 #define DEFAULT_CODEC_ID AV_NE(CODEC_ID_PCM_S16BE, CODEC_ID_PCM_S16LE)
 
+#define ALSA_BUFFER_SIZE_MAX 32768
+
 typedef struct {
     AVClass *class;
     snd_pcm_t *h;