Add test to verify fix for FreeBSD-SA-19:20.bsnmp

[darrick.freebsd_gmail.com]

Summary

Verify passing in improper length in asn_buf to asn_get_header() returns ASN_ERR_EOBUF.

Test Plan

Verified test passing with fix for FreeBSD-SA-19:20.bsnmp and failing without.

[cem]

lib/libbsnmp/tests/bsnmpd_test.c

10

The explicit length can be dropped to size it correctly automatically, char test_buf[] = { ... };

16

2, or sizeof(test_buf)

[darrick.freebsd_gmail.com]

Update per CR feedback.

[cem]

lib/libbsnmp/tests/bsnmpd_test.c

16

this one was missed?

[darrick.freebsd_gmail.com]

Update per CR feedback.