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.

source/blender/blenkernel/intern/deform.c

@@ -1557,7 +1557,7 @@ void BKE_defvert_blend_read(BlendDataReader *reader, int count, MDeformVert *mdv
     /*convert to vgroup allocation system*/
     MDeformWeight *dw;
     if (mdverts->dw && (dw = BLO_read_get_new_data_address(reader, mdverts->dw))) {
-      const ssize_t dw_len = mdverts->totweight * sizeof(MDeformWeight);
+      const size_t dw_len = MAX2(mdverts->totweight, 0) * sizeof(MDeformWeight);
       void *dw_tmp = MEM_mallocN(dw_len, __func__);
       memcpy(dw_tmp, dw, dw_len);
       mdverts->dw = dw_tmp;