Fix verifier error

[JoukoVirtanen]

Description

Fixes a verifier error, by adding null checks.

Checklist

• Investigated and inspected CI test results
• Updated documentation accordingly

Automated testing

• Added unit tests
• Added integration tests
• Added regression tests

If any of these don't apply, please comment below.

Testing Performed

Created an infra cluster with flavor gke-default. The kernel version was 6.8.0-1033-gke. There was the following error in the fact logs

[DEBUG 2025-11-13T01:09:09Z] (/cachi2/output/deps/cargo/aya-obj-0.2.1/src/relocation.rs:443) finished relocating program `trace_path_unlink` function `trace_path_unlink`
Error: the BPF_PROG_LOAD syscall failed. Verifier output: 0: R1=ctx() R10=fp0
; int BPF_PROG(trace_file_open, struct file* file) {
0: (79) r7 = *(u64 *)(r1 +0)
func 'bpf_lsm_file_open' arg0 has btf_id 602 type STRUCT 'file'
1: R1=ctx() R7_w=trusted_ptr_file()
1: (b7) r6 = 0                        ; R6_w=0
; unsigned int zero = 0;
2: (63) *(u32 *)(r10 -24) = r6        ; R6_w=0 R10=fp0 fp-24=????0
3: (bf) r2 = r10                      ; R2_w=fp0 R10=fp0
4: (07) r2 += -24                     ; R2_w=fp-24
; return bpf_map_lookup_elem(&metrics, &zero);
5: (18) r1 = 0xffff89a3c6a15c00       ; R1_w=map_ptr(map=metrics,ks=4,vs=80)
7: (85) call bpf_map_lookup_elem#1    ; R0_w=map_value_or_null(id=1,map=metrics,ks=4,vs=80)
; m->file_open.total++;
8: (79) r1 = *(u64 *)(r0 +0)
R0 invalid mem access 'map_value_or_null'
verification time 176 usec
stack depth 24
processed 8 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0


Caused by:
    Permission denied (os error 13)

After making the changes here deployed ACS again, and fact did not crash.

[Molter73]

These changes can also be found in #125, please verify if that PR works for the kernel you are working on and help get that PR merged, since it addresses a lot more than just these verifier issues.