pcm: use nullptr instead of NULL

src/pcm/PcmConvert.hxx

@@ -73,9 +73,9 @@ public:
 	 * @param src_size the size of #src in bytes
 	 * @param dest_format the requested destination audio format
 	 * @param dest_size_r returns the number of bytes of the destination buffer
-	 * @param error_r location to store the error occurring, or NULL to
+	 * @param error_r location to store the error occurring, or nullptr to
 	 * ignore errors
-	 * @return the destination buffer, or NULL on error
+	 * @return the destination buffer, or nullptr on error
 	 */
 	const void *Convert(const void *src, size_t src_size,
 			    size_t *dest_size_r,