fd-util: add ASSERT_FD() that is similar to ASSERT_PTR() but for fds

src/basic/fd-util.h

@@ -121,3 +121,11 @@ static inline char *format_proc_fd_path(char buf[static PROC_FD_PATH_MAX], int f
         format_proc_fd_path((char[PROC_FD_PATH_MAX]) {}, (fd))
 
 const char *accmode_to_string(int flags);
+
+/* Like ASSERT_PTR, but for fds */
+#define ASSERT_FD(fd)                           \
+        ({                                      \
+                int _fd_ = (fd);                \
+                assert(_fd_ >= 0);              \
+                _fd_;                           \
+        })