[libc][string] fix strncpy potential buffer overflow

The wrong placement of the increment for index `i` causes an unexpected
behavior, which the `strncpy` writes an extra '\0'.

For example:
The `src` string is "abc". The buffer size of `dest` is 5.

When we call `strncpy(dest, src, 5)`, the first `for` loop copies the
characters, 'a', 'b', and 'c', to the `dest[0:2]`. In the 4th iteration,
however, the `for` loop breaks due to the termination of `src` whereas
the value of `i` stays 3. At the moment, it has copied 4 bytes,
including the '\0' of `src`.

In the second `for` loop, we have `i = 3` and `count = 5`, so the loop
copies two more '\0' to the `dest`. As a result, the `strncpy` copies 6
bytes to the `dest` buffer, leading to buffer overflow.

Fix the issue by increasing the index `i` before every copy.

Signed-off-by: Cody Wong <codycswong@google.com>

lib/libc/string/strncpy.c

@@ -17,7 +17,7 @@ strncpy(char *dest, char const *src, size_t count) {
     char *tmp = dest;
 
     size_t i;
-    for (i = 0; i < count && (*dest++ = *src++) != '\0'; i++)
+    for (i = 0; i++ < count && (*dest++ = *src++) != '\0'; )
         ;
     for (; i < count; i++)
         *dest++ = '\0';