selftests/bpf: add missing pointer dereference for map stacktrace fixup

I get a segfault without it, other fixups always do dereference, and
without dereference I don't understand how it can ever work.

Fixes: 7c85c448 ("selftests/bpf: test_verifier, check bpf_map_lookup_elem access in bpf prog")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/test_verifier.c

@@ -14166,7 +14166,7 @@ static void do_test_fixup(struct bpf_test *test, enum bpf_map_type prog_type,
 		do {
 			prog[*fixup_map_stacktrace].imm = map_fds[12];
 			fixup_map_stacktrace++;
-		} while (fixup_map_stacktrace);
+		} while (*fixup_map_stacktrace);
 	}
 }