fix(pkg/rootless): fix get_cmd_line_args memleak in rootless_linux.

[FedeDP]

The assignment to NULL was preventing gcc cleanup attribute to auto-free the pointer for us, since it received a pointer to a NULL pointer.

Spotted using -fsanitize=address:

==1050100==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 512 byte(s) in 1 object(s) allocated from:
    #0 0x70591a2fd9c7 in malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:69
    #1 0x705915490406 in get_cmd_line_args /home/federico/go/pkg/mod/github.com/containers/podman/[email protected]/pkg/rootless/rootless_linux.c:313

Does this PR introduce a user-facing change?

None

[openshift-ci]

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[FedeDP]

Uh wrong analysis; will double check to understand where does the leak come from.
Sorry for the noise :/