More-robust svg_load_image fallback

Suggested by Alan Third (Bug#44655#56).
* src/image.c (svg_load_image): Fall back on
rsvg_handle_get_geometry_for_layer if the
rsvg_handle_get_intrinsic_dimensions computations yielded unusable
viewbox width and height, instead of falling back only if
rsvg_handle_get_intrinsic_dimensions did not report image width
and height, or did not report a viewbox.

src/image.c

@@ -10484,6 +10484,9 @@ svg_load_image (struct frame *f, struct image *img, char *contents,
       viewbox_height = viewbox.height;
     }
   else
+    viewbox_width = viewbox_height = 0;
+
+  if (viewbox_width == 0 || viewbox_height == 0)
     {
       /* We haven't found a usable set of sizes, so try working out
          the visible area.  */