pxar/create: fix endless loop for shrinking files

when a file shrunk during backup, we endlessly looped, reading/copying 0 bytes
we already have code that handles shrunk files, but we forgot to
break from the read loop

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

src/pxar/create.rs

@@ -661,6 +661,8 @@ impl<'a, 'b> Archiver<'a, 'b> {
             if got as u64 > remaining {
                 self.report_file_grew_while_reading()?;
                 got = remaining as usize;
+            } else if got == 0 {
+                break; // we reached eof
             }
             out.write_all(&self.file_copy_buffer[..got])?;
             remaining -= got as u64;