You just test:
if ( in->bv_len > MYSIZE || in->bv_len + len > MYSIZE )
return FAIL;
Except that in->bv_len + len can wrap around:-) In this case, use
if ( in->bv_len > MYSIZE - len ) since len will be <= MYSIZE.
No, you don't know whether len is <= MYSIZE, but you _do_ know that
in->bv_len is less than MYSIZE from the first clause in the test. So:
if ( in->bv_len > MYSIZE || len > MYSIZE - in->bv_len )
return FAIL;