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>