CLNP: squelch a fall-through warning (GH #618)

CLNP: squelch a fall-through warning (GH #618)

This change addresses one of the warnings listed in the bug report.

./print-isoclns.c: In function ‘clnp_print’:
./print-isoclns.c:1054:16: warning: this statement may fall through [-Wimplicit-fallthrough=]
             if (EXTRACT_U_1(pptr) == NLPID_CLNP) {
                ^
./print-isoclns.c:1061:9: note: here
         case  CLNP_PDU_DT:
         ^~~~

[skip ci]