FreeBSD -- Out of bound read in elf_note_prpsinfo()

Problem Description:
When dumping core and saving process information, proc_getargv()
might return an sbuf which have a sbuf_len() of 0 or -1, which is not
properly handled.
Impact:
An out-of-bound read can happen when user constructs a specially
crafted ps_string, which in turn can cause the kernel to crash.