LibCore: Populate the read buffer of Core::Stream::BufferedHelper

Previously, we weren't ever populating the read buffer in read(), which
was making the BufferedHelper useless, how silly :^). This introduces
a buffer refill when we have run out of buffered samples, restoring
FlacLoader performance from the new low of 200% (directly before this
commit) to the old level of ~1400%.

Userland/Libraries/LibCore/Stream.h

@@ -522,6 +522,11 @@ public:
         // Otherwise, let's try an extra read just in case there's something
         // in our receive buffer.
         auto stream_nread = TRY(stream().read(buffer.slice(buffer_nread)));
+
+        // Fill the internal buffer if it has run dry.
+        if (m_buffered_size == 0)
+            TRY(populate_read_buffer());
+
         return buffer_nread + stream_nread;
     }