Fix FrameSizeTestsLarge unit-test on 32-bit arch.

If the img allocation fails the test used to crash before on
32 bit architecture. This patch uses null check on img in
FillFrame. Also, if the first frame initialization has not been
conducted VPX_CODEC_ERROR is expected to return rather than
VPX_CODEC_OK.

Change-Id: I5c4e59c156374009012d280d6ff971a89b43c11f