cleanup: decrease the complexity of the d_decode_header() function
cleanup: decrease the complexity of the d_decode_header() function

Split out the part that parses the IR(-DYN) packets for the profile ID,
searches for the context with its CID, then creates a new context if needed.

This change makes GNU complexity happy :)

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