avcodec/smacker: Use same variable for return values and errors

avcodec/smacker: Use same variable for return values and errors

smacker_decode_header_tree() uses different variables for return values
(res) and for errors (err) leading to code like
res = foo(bar);
if (res < 0) {
    err = res;
    goto error;
}
Given that no positive return value is ever used at all one can simplify
the above by removing the intermediate res.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>