Fix NULL access segfault
https://bugzilla.redhat.com/show_bug.cgi?id=1071238
https://src.fedoraproject.org/rpms/gv/blob/rawhide/f/gv-bug1071238.patch

diff a/src/zoom.c b/src/zoom.c
--- a/src/zoom.c
+++ b/src/zoom.c
@@ -104,12 +104,12 @@ zoom_createZoom(Widget w _GL_UNUSED, XtP
 
     BEGINMESSAGE(zoom_createZoom)
 
-    if (!(gv_psfile != NULL || (gv_gs_arguments && *gv_gs_arguments))) {
+    if (!gv_psfile) {
        INFMESSAGE(no file) ENDMESSAGE(zoom_createZoom)
        return;
     }
 
-    if (strcmp(gv_filename, "-")) {
+    if (gv_filename != NULL && strcmp(gv_filename, "-")) {
        stat(gv_filename, &sbuf);
        if (mtime != sbuf.st_mtime) {
 	  INFMESSAGE1(file has changed) ENDMESSAGE1(zoom_createZoom)return;