port_fwd: Don't try to read bound ports from invalid file handles

This is a minimal fix for what would be reported by Coverity as
"Improper use of negative value" (CWE-394): port_fwd_init() doesn't
guarantee that all the pre-opened file handles are actually valid.

We should probably warn on failing open() and open_in_ns() in
port_fwd_init(), too, but that's outside the scope of this minimal
fix.

Before commit 5a0485425b ("port_fwd: Pre-open /proc/net/* files
rather than on-demand"), we used to have a single open() call and
a check after it.

Fixes: 5a0485425b ("port_fwd: Pre-open /proc/net/* files rather than on-demand")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

port_fwd.c

@@ -45,6 +45,9 @@ static void procfs_scan_listen(int fd, unsigned int lstate,
 	unsigned int state;
 	char *line;
 
+	if (fd < 0)
+		return;
+
 	if (lseek(fd, 0, SEEK_SET)) {
 		warn("lseek() failed on /proc/net file: %s", strerror(errno));
 		return;