mskd: vpp: error out gracefully instead of segfaulting if Init failed

mskd: vpp: error out gracefully instead of segfaulting if Init failed

Since we do the MSDK initializing in set_caps(), a FALSE
return may still cause the invokation of set_caps() again
and this will leads to buffer allocation and other mess-up.
So make sure the msdk initialized correctly before trying
to do any buffer allocation.

https://bugzilla.gnome.org/show_bug.cgi?id=796465