avpicture: Don't assume a valid pix fmt in avpicture_get_size

When called from the v4l2 input device, pix_fmt can be
AV_PIX_FMT_NONE (for jpeg formats). Before 50ba57e0, this wasn't
an issue for avpicture_get_size, but after that commit, this
lead to crashes.

Signed-off-by: Martin Storsjö <martin@martin.st>
