elf_note_prpsinfo: handle more failures from proc_getargv()

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

(cherry picked from commit 00d17cf342cd9f4f8fd1dcd79c8caec359145532)