Auto merge of #10048 - ehuss:curl-progress-panic, r=alexcrichton

Fix debug panic on download with redirect body.

With a debug build of cargo, downloading crates can panic if the download host issues a redirect with a body. From what I can see, the curl progress function gets called with the original size of the redirect body (such as total=154 cur=154, indicating that it has read 154 bytes of the redirect message). Then it calls the progress function again with cur=0 to start again from the beginning.  The next line in this patch, `cur - dl.current.get()` would panic since it is a `u64` and a 0 value of `cur` is less than the old `current`.

This was never really an issue with crates.io because it emits a redirect body of 0 bytes.

I think it is fine to skip this block in that situation, as it is only for resetting the timeout counter.  Though, I guess it could use `saturating_sub` instead.

Author: bors

src/cargo/core/package.rs

@@ -1006,7 +1006,7 @@ impl<'a, 'cfg> Downloads<'a, 'cfg> {
         let dl = &self.pending[&token].0;
         dl.total.set(total);
         let now = Instant::now();
-        if cur != dl.current.get() {
+        if cur > dl.current.get() {
             let delta = cur - dl.current.get();
             let threshold = self.next_speed_check_bytes_threshold.get();