Blender Git Loki

Cleanup: Fix build error with msvc

`ssize_t` is not a standardized type (it's a posix type)
given the line in question here is calculating the size
of a memory allocation there's no logical way this
should ever be negative.

I do not know this code too well and was unsure if
`mdverts->totweight` could ever be < 0, so I protected
it with a clamp, just in case.