sd-id128: check that the kernel is feeding us proper data

The characters are already checked, so we show that
we don't trust the kernel. Make sure we don't overrun
the buffer too.

src/libsystemd-id128/sd-id128.c

@@ -170,6 +170,9 @@ _public_ int sd_id128_get_boot(sd_id128_t *ret) {
         for (j = 0, p = buf; j < 16; j++) {
                 int a, b;
 
+                if (p >= buf + k)
+                        return -EIO;
+
                 if (*p == '-')
                         p++;