Fix issue when importing m4a

PBD::RingBuffer<>::rw_vector can return a NULL pointer
for the 2nd buffer, which can lead to undefined behavior:
`memcpy (NULL, src, 0);`

libs/ardour/ffmpegfileimportable.cc

@@ -236,6 +236,9 @@ FFMPEGFileImportableSource::did_read_data (std::string data, size_t size)
 		samplecnt_t written = 0;
 		for (int i = 0; i < 2; ++i) {
 			samplecnt_t cnt = std::min<samplecnt_t> (n_samples, wv.len[i]);
+			if (!cnt || !wv.buf[i]) {
+				break;
+			}
 			memcpy (wv.buf[i], cur, cnt * sizeof (float));
 			written += cnt;
 			n_samples -= cnt;