selinux: free str on error in str_read()

In [see "Fixes:"] I missed the fact that str_read() may give back an
allocated pointer even if it returns an error, causing a potential
memory leak in filename_trans_read_one(). Fix this by making the
function free the allocated string whenever it returns a non-zero value,
which also makes its behavior more obvious and prevents repeating the
same mistake in the future.

Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1461665 ("Resource leaks")
Fixes: c3a2761 ("selinux: optimize storage of filename transitions")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/ss/policydb.c

@@ -1035,14 +1035,14 @@ static int str_read(char **strp, gfp_t flags, void *fp, u32 len)
 	if (!str)
 		return -ENOMEM;
 
-	/* it's expected the caller should free the str */
-	*strp = str;
-
 	rc = next_entry(str, fp, len);
-	if (rc)
+	if (rc) {
+		kfree(str);
 		return rc;
+	}
 
 	str[len] = '\0';
+	*strp = str;
 	return 0;
 }