mbedtls_buffer_offset() function

Return an offset into a buffer. This is just the addition of an offset to a pointer, except that this function also accepts an offset of 0 into a buffer whose pointer is null. (`p + n` has undefined behavior when `p` is null, even when `n == 0`. A null pointer is a valid buffer pointer when the size is 0, for example as the result of `malloc(0)` on some platforms.)