[non io][non-BE] Resolve "Crash in OSBA encder with ltv signal"

• Related issues: #871 (closed)
• Requested reviewers:

Reason why this change is needed

Crash when ACELP core runs out of bits with a specific OSBA test vector

Description of the change

reduce maximum number of DirAC metadata bits

Affected operating points

SBA and OSBA at 24.4 kbps

• bitstream is not BE
• decoder is not modified, so backward compatibility should be given
• output is not BE