[non-BE] Resolve "MD coding in ISM inactive frames"

Related to #532 (closed)

A crash was observed when coding 2ISM at 64 kbps when both objects are inactive:

Frames processed:       29
Internal fatal error: Error: Too high bitrate (31350 bps) for ACELP@12k8 in frame 29. Exiting!

In function: config_acelp1(), c:\myprojects_ivas\git_3gpp\ivas-codec\lib_com\bits_alloc.c:584

The fix is pushed within FIX_532_ISM_MD_INACTIVE. Non-BE outputs are expected: they are sporadical segments and improve the quality.