input/curl: ignore ResponseBoundary() while seeking

While seeking, metadata must not be updated.  ResponseBoundary() was
added in MPD 0.19.1, but I forgot to add the IsSeeking() check there.
This caused the "seekable" flag to reset.

NEWS

@@ -3,6 +3,8 @@ ver 0.19.3 (not yet released)
   - fix "(null)" result string to "list" when AlbumArtist is disabled
 * database
   - upnp: fix breakage due to malformed URIs
+* input
+  - curl: another fix for redirected streams
 * decoder
   - audiofile: fix crash while playing streams
   - ffmpeg: support opus

src/input/plugins/CurlInputPlugin.cxx

@@ -610,6 +610,10 @@ CurlInputStream::ResponseBoundary()
 	/* undo all effects of HeaderReceived() because the previous
 	   response was not applicable for this stream */
 
+	if (IsSeekPending())
+		/* don't update metadata while seeking */
+		return;
+
 	seekable = false;
 	size = UNKNOWN_SIZE;
 	ClearMimeType();