evas_vg_load_svg: Fix negative attrs length

Summary:
After finding no attributes but spaces, attrsLength could be negative.
This will cause a segfault in parser functions.
So, change the position of attrs_length to prevent this.

Test Plan:
Example SVG
```
<?xml version="1.0" encoding="UTF-8"?>
<svg><g  ></g></svg>
```

Reviewers: Hermet, raster, kimcinoo

Reviewed By: Hermet

Subscribers: #reviewers, cedric, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D12314

src/modules/evas/vg_loaders/svg/evas_vg_load_svg.c

@@ -2276,12 +2276,12 @@ _evas_svg_loader_xml_open_parser(Evas_SVG_Loader *loader,
      {
         // find out the tag name starting from content till sz length
         sz = attrs - content;
-        attrs_length = length - sz;
         while ((sz > 0) && (isspace(content[sz - 1])))
           sz--;
         if ((unsigned int)sz >= sizeof(tag_name)) return;
         strncpy(tag_name, content, sz);
         tag_name[sz] = '\0';
+        attrs_length = length - sz;
      }
 
    if ((method = _find_group_factory(tag_name)))