Backport from lib2geom: Fix crash in itemBounds (LP#1796046)

Refs: lib2geom@f86669bd
Refs: https://bugs.launchpad.net/inkscape/+bug/1796046

src/2geom/pathvector.cpp

@@ -172,6 +172,7 @@ public:
 
     Interval itemBounds(ItemIterator ii) {
         OptRect r = ii->path->boundsFast();
+        if (!r) return Interval();
         return (*r)[X];
     }