hevc: make avcodec_decode_video2() fail if get_format() fails

hevc: make avcodec_decode_video2() fail if get_format() fails

Personally, I need the decoder to back out if get_format() returns no
usable pixel format. This didn't work because the error code was not
propagated down the call chain. This in turn happened because the
variable declaration removed in this patch shadowed the variable, whose
value is returned at the end of the function. Consequently, failures of
decode_nal_unit() were ignored in this place.

Reviewed-by:  Andreas Cadhalpun <andreas.cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>