Fix a buffer overflow in bootparamd(8)

If /etc/bootparams contains a line with an excessively long pathname, and a
client asks for that path, then bootparamd will overflow a buffer and crash
while parsing that line. This is not remotely exploitable since it requires
a malformed /etc/bootparams file.

Reported by: Coverity
CID: 1305954
MFC after: 1 week
Sponsored by: Spectra Logic Corp