QIODevice::seek(): remove unnecessary check

At this point, the buffer is always not empty, as otherwise the offset
could not be smaller than it.

Change-Id: Iec04c0463623c4ed1e86bbcba2240653f110e315
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

src/corelib/io/qiodevice.cpp

@@ -670,7 +670,7 @@ bool QIODevice::seek(qint64 pos)
         // operation will then refill the buffer. We can optimize this, if we
         // find that seeking backwards becomes a significant performance hit.
         d->buffer.clear();
-    else if (!d->buffer.isEmpty())
+    else
         d->buffer.skip(offset);
 
 #if defined QIODEVICE_DEBUG