Enhance the rohc_decompress() function:  - replace an error message by an assertion in an internal 'should not happen'    case,  - add some comments in the rohc_decompress() function.
Enhance the rohc_decompress() function:
 - replace an error message by an assertion in an internal 'should not happen'
   case,
 - add some comments in the rohc_decompress() function.

didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc