ftrace: Fix invalid address access in lookup_rec() when index is 0

In the Linux kernel, the following vulnerability has been resolved:

ftrace: Fix invalid address access in lookup_rec() when index is 0

KASAN reported follow problem:

BUG: KASAN: use-after-free in lookup_rec

Read of size 8 at addr ffff000199270ff0 by task modprobe

CPU: 2 Comm: modprobe

Call trace:

kasan_report

__asan_load8

lookup_rec

ftrace_location

arch_check_ftrace_location

check_kprobe_address_safe

register_kprobe

When checking pg->records[pg->index - 1].ip in lookup_rec(), it can get a

pg which is newly added to ftrace_pages_start in ftrace_process_locs().

Before the first pg->index++, index is 0 and accessing pg->records[-1].ip

will cause this problem.

Don't check the ip when pg->index is 0.