#1554 clangd_client : fix crash on completion response after project closed

[Christo]

Crash was seen once when I was switching projects after startup. Coredump indicated parser was null

#2  0x0000793cc8fc75c7 in Parser::OnLSP_DiagnosticsResponse (this=0x0, event=...)
#3  0x0000793cc8f131aa in ClgdCompletion::OnLSP_Event (this=0x1b429260, event=...)

Attaching patch to ignore completion response of parser is null.

[pecan]

Applied, rev 13734, thanks Christo