Require non-negative length in stream_get_contents()

Require non-negative length in stream_get_contents()

If the length is not -1, require it to be non-negative.

Using such lengths doesn't make sense (as only -1 is special-case
to read in chunks, anything else will end up doing a huge upfront
allocation) and can lead to string allocation overflow.

A similar check is already in place for file_get_contents(). That
one does not allow -1 (and uses null instead), but this function
is explicitly specified to accept -1, so stick to that behavior.