mediafile._safe_cast: be safer when converting to int
mediafile._safe_cast: be safer when converting to int

The regex «[\+-]?[0-9]*» possibly matches a single minus/plus, which would then
be passed on to int(), raising a ValueError from within _safe_cast. The test
suite covered this for float, but not for int.

We now make sure we actually have a number after the sign by using a Kleene
plus.

beetbox/mediafile