media: venus: dec: Handle the case where find_format fails

Debugging the decoder on msm8916 I noticed the vdec probe was crashing if
the fmt pointer was NULL.

A similar fix from Colin Ian King found by Coverity was implemented for the
encoder. Implement the same fix on the decoder.

Fixes: 7472c1c6 ("[media] media: venus: vdec: add video decoder files")
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>