pcm_buffer: pcm_buffer_get() never returns NULL

This fixes a bug when libsamplerate returns an empty buffer for a very
small input buffer.  The caller thinks this is an error, bug there is
no GError object.

src/pcm_buffer.c

@@ -34,6 +34,11 @@ pcm_buffer_get(struct pcm_buffer *buffer, size_t size)
 {
 	assert(buffer != NULL);
 
+	if (size == 0)
+		/* never return NULL, because NULL would be assumed to
+		   be an error condition */
+		size = 1;
+
 	if (buffer->size < size) {
 		/* free the old buffer */
 		g_free(buffer->buffer);