icmp6 fix no respond to neighbor solicit message

The structure size used in grub_netbuff_pull to get the pointer to
option header is apparently wrong, which leads to subsequent range check
failed and therefore not responding to any neighbor solicit message in my
testing.
efi: check *path non-null before grub_strrchr

The EFI version of grub_machine_get_bootlocation crops the boot image
name back to the last / in order to get a directory path. However, it
does not check that *name is actually set before calling grub_strrchr
to do this, and neither does grub_strrchr before dereferencing a NULL
pointer.

Parent function, grub_set_prefix_and_root, does check the pointer
before using.
Fix date in last ChangeLog entry
grub-fs-tester: consistently print output of grub ls if test fails