Fix SVG metadata handler by changing offset from -1 to 0

Fix SVG metadata handler by changing offset from -1 to 0

Starting in php 7.1, negative offsets are supported in
file_get_contents. Previously they were not (and in fact, the
php docs said -1 was the default value for the offset if unspecified).

The result of this, is starting in php 7.1, MediaWiki could not determine
the aspect ratio of SVG files that were larger than 256kb.

Follows up e6de99be

Bug: T213501
Change-Id: I565e51cd8131542b9a70da49e9cc36c5594ecda3