These 2 fields are not always the same, it is simpler to always use the same field
for detecting studio profile

Fixes: null pointer dereference
Fixes: ffmpeg_crash_3.avi

Found-by: Thuan Pham <thuanpv@comp.nus.edu.sg>, Marcel Böhme, Andrew Santosa and Alexandru RazvanCaciulescu with AFLSmart
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>