Fix integer overflow in utf{16,32}_to_utf8_length

Without an explicit check, the return value can wrap around and return
a value that is far too small to hold the data from the resulting
conversion.

No CTS test is provided because it would need to allocate at least
SSIZE_MAX / 2 bytes of UTF-16 data, which is unreasonable on 64-bit
devices.

Bug: 37723026
Test: run cts -p android.security
Change-Id: I56ba5e31657633b7f33685dd8839d4b3b998e586