Don't do concatenation with strncat; the length parameter in strncat
is only for how many chars to append at most, not for the full output
buffer size. To safely use strncat, one would have to do
strncat(buf, str, sizeof(buf) - strlen(buf)).
By using snprintf, we guarantee that the buffer is null terminated, and
we don't need to use strnlen at all.
(If compatibility with older MSVC versions that lack snprintf, one
can use _snprintf instead and manually add the null termination.)
Change-Id: I1c2322c7a406ddd5e6551a96c460da60deeffda1
