Updating from an older 8.2-Stable to 8.4-Stable, after running /usr/local/sbin/pkg2ng,  pkg immediately seg faults:
Converting packages from /var/db/pkg
Converting en-freebsd-doc-20110110...
Child process pid=21252 terminated abnormally: Segmentation fault: 11

After building again with debug symbols, gdb provides the following stack trace:
#0  0x000000080175412a in strlcpy () from /lib/libc.so.7
#1  0x00000008006df44b in plist_new (pkg=0x801a9c800, stage=0x0) at pkg_ports.c:1083
#2  0x00000008006dfd14 in ports_parse_plist (pkg=0x801a9c800,
    plist=0x7fffffffdec0 "/var/db/pkg/en-freebsd-doc-20110110/+CONTENTS", stage=0x0) at pkg_ports.c:1140
#3  0x000000080070cd34 in pkg_old_load_from_path (pkg=0x801a9c800, path=0x7fffffffe380 "/var/db/pkg/en-freebsd-doc-20110110")
    at pkg_old.c:72
#4  0x000000000040e20d in convert_from_old (pkg_add_dbdir=0x425b3d "/var/db/pkg", dry_run=false) at convert.c:308
#5  0x000000000040e471 in exec_convert (argc=0, argv=0x7fffffffeaf8) at convert.c:368
#6  0x00000000004144d9 in main (argc=1, argv=0x7fffffffeaf0) at main.c:822

In frame 1:
1083            strlcpy(p->prefix, prefix, sizeof(p->prefix));

prefix is a null pointer, so the strlcpy seg faults.  This suggests that the previous line:
        pkg_get(pkg, PKG_PREFIX, &prefix);
Set &prefix to a null pointer, which was unchecked before the strlcpy.  As a result no errors are provided, no warnings
offered, no workarounds are performed, and the program crashes instead of handling whatever problem exists gracefully.

It appears to be trying to work with "/var/db/pkg/en-freebsd-doc-20110110/+CONTENTS" at the time of the crash.

How-To-Repeat:
Have an older ports db. Try to upgrade.

Fix:
Not known at this time.