drm_hwcomposer: Return error from GetEdidBlob if blob is null
drmModeGetPropertyBlob can return NULL blobs resulting in a segfault in GetDisplayIdentificationData which expects blob to be set to a valid pointer when GetEdidBlob returns 0.
Fixes: adc5d822
I haven't read much into the DRM API yet - is it weird/unexpected for a connector to have the EDID
property without any data attached to it?