wasip3: Improve/fix handling of buffered reads (#793)

This updates the code path that performs a buffered read for a stream
when zero-length reads/writes are determined to not signal readiness
correctly. Specifically a defensive assertion that this is a
nonzero-length read is added and additionally in the case that I/O
completes immediately the return code is handled appropriately.
Unfortunately I don't know of a way to test this as it's not testable
from Wasmtime today and the file streams implementation in Wasmtime ends
up sort of accidentally respecting the zero-length protocol (sort of).
Thus, for now, this is just a code change until more comprehensive
testing such as #766 is implemented.

Author: alexcrichton

libc-bottom-half/sources/file_utils.c

@@ -238,6 +238,7 @@ static int wasip3_read_start_nonzero(wasip3_io_state_t *state, size_t length) {
   assert(!(state->flags & WASIP3_IO_ZERO_INPROGRESS));
   assert(!(state->flags & WASIP3_IO_DONE));
   assert(state->buf == NULL);
+  assert(length);
   state->buf = malloc(length);
   if (!state->buf) {
     errno = ENOMEM;
@@ -252,7 +253,7 @@ static int wasip3_read_start_nonzero(wasip3_io_state_t *state, size_t length) {
     errno = EWOULDBLOCK;
     return -1;
   }
-
+  state->buf_end = wasip3_io_update_code(state, status);
   return 0;
 }