vpx_image: clear user provided vpx_image_t early

this avoids uninitialized values and potential misuse of them which
could lead to a crash should the function fail

this is the same fix that was applied in libaom:
d0cac70b5 Fix a free on invalid ptr when img allocation fails

Bug: webm:1722
Change-Id: If7a8d08c4b010f12e2e1d848613c0fa7328f1f9c
1 file changed