MFC r311750,r311754,r311757:

r311750:

Check result from smiGetFirstNode and smiGetNodeByOID

This avoids a segfault with malformed or unanticipated files,
like IPV6-TC.txt (a file containing just TEXTUAL-CONVENTIONS).

Found with: gensnmpdef /usr/local/share/snmp/mibs/IPV6-TC.txt

r311754:

Use calloc instead of malloc + memset(.., 0, ..)

r311757:

Similar to r311750, check for the result from smiGetModule to avoid a segfault
when dereferencing a NULL pointer later on.

Choose to just check for the NULL pointer in the next for-loop for now to fix
the issue with a minimal amount of code churn

sys/queue.h use here would make more sense than using a static table