elf_note_prpsinfo: handle more failures from proc_getargv()

Resulting sbuf_len() from proc_getargv() might return 0 if user mangled
ps_strings enough. Also, sbuf_len() API contract is to return -1 if the
buffer overflowed. The later should not occur because get_ps_strings()
checks for catenated length, but check for this subtle detail explicitly
as well to be more resilent.

The end result is that p_comm is used in this situations.

Approved by: so
Security: FreeBSD-SA-22:09.elf
Reported by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>
Reviewed by: delphij, markj
admbugs: 988
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35391