#1390 Fix for crash in swinst_pkginfo

[Narendra Kumar S S]

This is a patch for https://sourceforge.net/p/net-snmp/bugs/2930/
snmpd on Solaris 11 crashes while querying for hrSWInstalled and one of the installed packages is missing CATEGORY field

[Bart Van Assche]

This patch has been committed on the v5.8 and master branches. Thanks for the patch!