error out early if setup fails and thus the ext2 block reader returns EOF

mmdebstrap

@@ -5920,7 +5920,13 @@ sub main() {
     my $numblocks = 0;
     close $nblkwriter;
     if (!$options->{dryrun} && $format eq 'ext2') {
-        chomp($numblocks = <$nblkreader>);
+        $numblocks = <$nblkreader>;
+        if (!defined $numblocks) {
+            # This can happen if the setup process died early and thus closes
+            # the pipe from the other and. The EOF is turned into undef.
+            error "failed to read required number of blocks";
+        }
+        chomp $numblocks;
     }
     close $nblkreader;