basic/copy: Avoid pointless zero-sized sendfile/copy

A while ago I copied a version of this code into libglnx, and
was recently doing some work on it:
GNOME/libglnx#44

While there I noticed that if a maximum size was provided, we could
end up doing a zero-sized `sendfile()` call which is obviously pointless.
If we have nothing to do at the of the loop, just break out.

src/basic/copy.c

@@ -172,6 +172,8 @@ int copy_bytes(int fdf, int fdt, uint64_t max_bytes, CopyFlags copy_flags) {
                  * but don't go below our copy buffer size, unless we are
                  * close the limit of bytes we are allowed to copy. */
                 m = MAX(MIN(COPY_BUFFER_SIZE, max_bytes), m - n);
+                if (m == 0)
+                        break;
         }
 
         return 0; /* return 0 if we hit EOF earlier than the size limit */