sandbox: don't call os_close with invalid file descriptor

If open() fails it returns -1. Calling close() with this value
makes no sense. Return -EIO instead.

Addresses-Coverity-ID: 185828 Improper use of negative value
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Fixes: 566bf3a869 ("sandbox: Add a function to read a host file")
Reviewed-by: Sean Anderson <seanga2@gmail.com>

arch/sandbox/cpu/os.c

@@ -188,7 +188,7 @@ int os_read_file(const char *fname, void **bufp, int *sizep)
 	fd = os_open(fname, OS_O_RDONLY);
 	if (fd < 0) {
 		printf("Cannot open file '%s'\n", fname);
-		goto err;
+		return -EIO;
 	}
 	size = os_filesize(fd);
 	if (size < 0) {