snmplib: Improve handling of zero-length ASN OCTET STRINGs (#2163)

snmplib: Improve handling of zero-length ASN OCTET STRINGs (#2163)

When converting zero-length objects to c-strings and terminating their
values, do not write to c-string storage buffer at offset `-1`.

Also reject 128-byte SNMP community values because our storage buffer
does not allow for their zero-termination. Such values were probably
silently truncated to 127 bytes.

Also reject SNMP community values with ASCII NUL characters because some
of our code does not support such values.