Fix possible nullptr dereferences in toxav/msi.c

Fixes two possible dereferences of null pointers in toxav/msi.c

The first is just reordering a dereference after actually checking the pointer's non-nullity, the second adds an extra if in (as far as I know) cold code to check for non-nullity.

In both cases there already are some nullity checks for those pointers, so I assumed that they can actually be null (otherwise those checks would be redundant).