dtrace: ensure return value of access_process_vm() is > 0

dtrace: ensure return value of access_process_vm() is > 0

If access_process_vm() returns a value <= 0, the loop intended to replace
NULs with whitespace in the ps string could turn into a memory sprayer
by incrementing its loop counter all the way to (uintptr_t)-1. This
often shows up as crashes with PTEs showing 0x20 stuffed inside, e.g.:

[  630.136149] PGD 66f7f067 PUD 802a5067 PMD 18fff0067 PTE 2020200208ad2025
[  630.136194] Bad pagetable: 000d [#1] SMP

[ 1698.220469] PGD 5cebd067 PUD 5ced6067 PMD 17fe6d067 PTE 8020202032652867
[ 1698.220511] Bad pagetable: 000d [#1] SMP

[  438.486319] PGD 136cee067 PUD 136d9b067 PMD 13ba2067 PTE 802020012c8bf867
[  438.486364] Bad pagetable: 000d [#1] SMP

though it can cause other crashes. To fix, bracket the loop logic in
a test of access_process_vm()'s return value.

Orabug: 22295336

Signed-off-by: Todd Vierling <todd.vierling@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>