valgrind: logic for last-ditch svg detection is busted

bIsGZip was never set, and the condition that uses it is additionally inverted

Change-Id: I0496bb27435c4323d74c1b99467d3ede68e7bee6
(cherry picked from commit 631378fe110ec7c4161a6c36011640522b881c27)
Reviewed-on: https://gerrit.libreoffice.org/12382
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>