kde5backend.cxx: Fix inverted logic leading to invalid write

Only write to 'pFakeArgv[2]' when 'aDisplay' is NON-empty.

Otherwise, the 'pFakeArgv' array only has size 2 and the write
is invalid.

(Found by valgrind while looking at another issue.)

Change-Id: I58aff6d25c8647bc6ef346af8ac09b0b0fc030b8
Reviewed-on: https://gerrit.libreoffice.org/60476Reviewed-by: Julien Nabet <serval2412@yahoo.fr>
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

shell/source/backends/kde5be/kde5backend.cxx

@@ -159,7 +159,7 @@ void initQApp()
 
     pFakeArgv[0] = strdup(getExecutable().getStr());
     pFakeArgv[1] = strdup("--nocrashhandler");
-    if (aDisplay.isEmpty())
+    if (!aDisplay.isEmpty())
         pFakeArgv[2] = strdup(aDisplay.getStr());
 
     char* session_manager = nullptr;