1. Check rax validity in more places in stream_family.cc
2. Cover streamTrim function and check lp validity  following the assert-fail in the tests.
3. Move raxStop to bottop of StreamAppendItem function though it's not clear that raxStop can
   actually cause the bug as ri.key is always 16 bytes and is never allocated on heap.

Addresses #6409 though not necessary fixes it.

Signed-off-by: Roman Gershman <roman@dragonflydb.io>