fix: set 'elf' to nil after closing in BPFLoadObject for GC

[rscampos]

In the BPFLoadObject function, the 'elf' field is closed, but it is not set to nil. Setting it to nil is necessary for it to be freed by the garbage collector.

[rscampos]

In order to test this commit, use the following snip before and after BPFLoadObject():

var memBefore runtime.MemStats
runtime.ReadMemStats(&memBefore)
fmt.Println("Alloc mem before: ", memBefore.Alloc)

Some results:
Without this commit:

sudo ./run.sh main-static
Memory stats before:
Alloc = 85952 bytes
Memory stats after:
Alloc = 85984 bytes

With this commit:

sudo ./run.sh main-static
Memory stats before:
Alloc = 85952 bytes
Memory stats after:
Alloc = 80144 bytes

[geyslan]

Good catch. ❤️ It seems safe to nullify it since m.loaded is checked first in the related logics.