The length by definition needs to include the 4 bytes for the size.
If a size of zero is provided this can result in an infinite loop as providing the zero to `src[l:]` will result in the slice not being consumed.  Allowing the zero length to be read for infinity.

…tput

In addition tot he MongoDB protocol Snappy also has an encoded size.
If the Snappy payload is larger (or at least claims to be larger), the Snappy module will allocate a new byte slice to match the needed size.
This could result in memory exhaustion.

Co-authored-by: Alan Parra <[email protected]>

Co-authored-by: Qingyang Hu <[email protected]>