EMI: Fix initialization of textures

Residualvm crashes when leaving the campfire szene on Knutting Town due
to a NULL-pointer dereference.

When initializing the texture with dummy values due to a missing tex
file, set _data to NULL so that a free doesn't cause any harm when
destructing the MaterialData objects.
Merge pull request #787 from chkr-private/material-fixes

EMI: Fix initialization of textures