input/cache/Stream: pass the correct mutex to InputCacheItem::Read()

This deadlock bug has been present ever since the very first commit
5d74b5c which added the input cache.

Author: MaxKellermann

NEWS

@@ -1,4 +1,6 @@
 ver 0.24.10 (not yet released)
+* input
+  - cache: fix deadlock bug
 * database
   - upnp: allow building with libupnp 1.14.30 which has fixed the API breakage
 * Windows

src/input/cache/Stream.cxx

@@ -64,9 +64,9 @@ CacheInputStream::Read(std::unique_lock<Mutex> &lock,
 		   #lock) and lock #InputCacheItem's mutex (from
 		   #BufferingInputStream) instead*/
 		const ScopeUnlock unlock{lock};
-		const std::lock_guard protect{i.mutex};
+		std::unique_lock cache_lock{i.mutex};
 
-		nbytes = i.Read(lock, _offset, dest);
+		nbytes = i.Read(cache_lock, _offset, dest);
 	}
 
 	offset += nbytes;