lib: Fix some logic/error-checking code

lib: Fix some logic/error-checking code

Using `MAX(0, $x)` here is useless since we're comparing against an
unsigned integer. Just unpack this and only subtract if it's safe to do
so.

Also, explicitly check for `fd >= 0` rather than just `!= -1` to be sure
it's a valid fd. And finally, explicitly check the return value of
`g_input_stream_read_all` as is done everywhere else in the tree and
make it clear that we're purposely ignoring the return value of `_flush`
here, but not in other places.

Discovered by Coverity.

Closes: #1692
Approved by: cgwalters