better fix for macos/freebsd, where writing to /dev/null does set errno, but also fwrite returns the required amount of bytes

vlib/os/file.c.v

@@ -342,8 +342,10 @@ pub fn (mut f File) write_full_buffer(buffer voidptr, buffer_len usize) ! {
 				if cerror == C.EINTR {
 					continue
 				}
+				if x != buffer_len {
 					return error(posix_get_error_msg(cerror))
 				}
+			}
 			if x <= 0 {
 				return error('C.fwrite returned 0')
 			}