pcm_resample: use pcm_buffer (1/2)

Return a temporary buffer from pcm_resample_*() and pcm_convert().

src/pcm_resample.c

@@ -23,4 +23,6 @@
 void pcm_resample_init(struct pcm_resample_state *state)
 {
 	memset(state, 0, sizeof(*state));
+
+	pcm_buffer_init(&state->buffer);
 }