Fix assertion when encountering invalid header line.

Fix assertion when encountering invalid header line.

The server test sets an empty identifier which results in a
headerLine containing "Server: ". The split then yields a list
with a single entry ("Server"). Calling [1] on it asserts then.

QFATAL : tst_QWebSocketServer::tst_connectivity()
 ASSERT failure in QList<T>::operator[]: "index out of range", file
/ssd/milian/projects/compiled/qt5/include/QtCore/qlist.h, line 476

Change-Id: I3e5c4750b304b2a4a5669a39e2d65a6b3e4e99cf
Reviewed-by: Kurt Pattyn <pattyn.kurt@gmail.com>