libbpf: don't close(-1) in multi-uprobe feature detector

Guard close(link_fd) with extra link_fd >= 0 check to prevent close(-1).

Detected by Coverity static analysis.

Fixes: 04d939a2ab22 ("libbpf: detect broken PID filtering logic for multi-uprobe")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20240529231212.768828-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

src/features.c

@@ -393,7 +393,8 @@ static int probe_uprobe_multi_link(int token_fd)
 	err = -errno; /* close() can clobber errno */
 
 	if (link_fd >= 0 || err != -EBADF) {
-		close(link_fd);
+		if (link_fd >= 0)
+			close(link_fd);
 		close(prog_fd);
 		return 0;
 	}