fix(c-api) Don't drain the entire captured stream when reading a small range.

We use `VecDeque::drain` to read the captured stream, zipped with the
given buffer. We could expect that only the yielded items from the
`drain` will be removed, but actually no. Reading [the
documentation](https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.drain):

> Note 1: The element `range` is removed even if the iterator is not
> consumed until the end.

So by using a range like `..` will drain the entire captured stream,
whatever we read from it. Said differently, if the given buffer length
is smaller than the captured stream, the first read will drain the
entire captured stream.

This patch fixes the problem by specifying a better range:
`..min(inner_buffer.len(), oc.buffer.len())`.

With this new range, it's actually useless to increment
`num_bytes_written`, we already know ahead of time the amount of bytes
we are going to read. Consequently, the patch simplifies this code a
little bit more.

lib/c-api/src/wasm_c_api/wasi/mod.rs

@@ -13,6 +13,7 @@ use super::{
 // required due to really weird Rust resolution rules for macros
 // https://github.com/rust-lang/rust/issues/57966
 use crate::error::{update_last_error, CApiError};
+use std::cmp::min;
 use std::convert::TryFrom;
 use std::ffi::CStr;
 use std::os::raw::c_char;
@@ -275,12 +276,16 @@ pub unsafe extern "C" fn wasi_env_read_stderr(
 
 fn read_inner(wasi_file: &mut Box<dyn WasiFile>, inner_buffer: &mut [u8]) -> isize {
     if let Some(oc) = wasi_file.downcast_mut::<capture_files::OutputCapturer>() {
-        let mut num_bytes_written = 0;
-        for (address, value) in inner_buffer.iter_mut().zip(oc.buffer.drain(..)) {
+        let total_to_read = min(inner_buffer.len(), oc.buffer.len());
+
+        for (address, value) in inner_buffer
+            .iter_mut()
+            .zip(oc.buffer.drain(..total_to_read))
+        {
             *address = value;
-            num_bytes_written += 1;
         }
-        num_bytes_written
+
+        total_to_read as isize
     } else {
         -1
     }